I'm writing out my adventures in completing this in the hopes that it will help others that stumble at this point in the game. If you did it another way then that's great! This worked for me and is a fairly simple step by step process that others can use to follow. Car names and minor details may not be 100% accurate as I'm at work right now and unable to peek at my garage.
I'll start off with the assumption that you the reader haven't done the GT World championship yet, and don't have the shiny Bugatti Veyron to play with. You can use whatever you wish to beat the GT World championship, I myself used my RX-7 Bathurst for it. The first thing I would suggest doing, would be getting your B-Spec driver up to level 7-8ish. When he has reached the appropriate level, run over to the Mazda dealership and buy the Roadster RS (NC) '07. Take it to the tuneup garage, put sports hard tires on it, and waste about 90,000ish credits bringing the horsepower on it up to around 260ish. Send your B-Spec driver into the Mazda Roadster cup and babysit the goober to victory. My driver Z. "Smell My Finger" Brennan was able to pull it off without too much trouble with 100hp more than the competition and good tires. You can probably get away with less tuning and more babysitting, but that car definitely scored victory for me.
Once your nincompoop driver is able to win that event, you will be presented with a nice Mazda Furai Concept car! Wooo! This car is stupidly light and with a little bit of extra downforce applied corners like it is glued to the road. Spend some money tuning it up if you wish (I did), and you now have a car that can absolutely SHRED the GT World championship. Feel free to use whatever car for the GT World cup you wish though, if you think you can do it with a kubelwagen, 4 fire extinguishers and some duct tape then go for it! After winning the Veyron, I dropped a bunch of money buying every tuning part for it I could get my hands on, and bumped the top speed up to 400ish kph. Personally, I actually found the Bugatti fairly easy to drive but that could just boil down to my preferred driving style.
As of right now, you have a few choices, you can EXP up for the 'Gone Like the Wind' races to get your race car, or you can attempt the All Stars race with what you have. If you want to hold off and do 'Gone Like the Wind' first, then I would suggest using the Veyron for the first race, and the Furai for the second race. I was able to destroy the race cars in the first race using the Veyron by about 5 seconds due to it just being so stupidly fast. For the corners, as long as you slow down early enough and follow the racing line, you'll get through without much trouble. Around the high speed oval is really where the Bugatti shines and you'll be able to put LOTS of distance between you and the race cars. For the second race, the Veyron will get absolutely SHREDDED due to how much it has to slow down to make the corners, and the straightaways not being long enough to properly catch up. The Furai on the other hand seems purpose built to annihilate this race, bump its top speed up to 400ish kph. I was able to get consistent 44 second lap times on indy using the Furai without much trouble at all. The trick in that race, is yes the race cars will dominate you on the first straightaway, but you'll FLY by them on the corners, as they have to slow down but the Furai NEVER has to touch the brakes for this race. After the first corner you should be able to draft one of the race cars to pick up speed. By lap 2, you should be in the lead, and the cornering ability of the car should make it so that catching you is absolutely HOPELESS. A few laps later and hooray! You have your very own Minolta 88C-V '89 race car! At this point you should be able to buy the remaining upgrades for it and use that to shred the All Stars race.
Now, if you're like me and don't want to wait for some sissy nancy race car to do the All Stars race, or if you just flat out enjoy driving the Veyron and want to use it. It is definitely possible to win the All Stars series with it! I managed to place 1st in the first 4 races, and 4th in the last (it was 1am when I did the last race, so I MIGHT have been able to place better if I was more awake). You MIGHT be able to do the All Stars race in the Furai, I haven't tried it yet, but you will need to be very good at drafting on the straights to overcome the power advantage of the race cars. Now, if you want to try out All Stars with the Veyron there are a few things you need to keep in mind:
A) The Veyron has terrible downforce, so you
WILL be taking air if you're not careful.
B) All of the races start from a dead stop, the Veyron has STUPIDLY good off the line power.
Use it!
C) If you have a hard time controlling the Veyron, turn on the racing line and follow it. It works!
D) If you go on the grass, you're going to be introduced to Mr. Wall very soon. Don't do it!
E) You need to know the tracks! If you're not running clean laps, you're going to get passed.
F) If you get passed by one of the nastier race cars on a turn-heavy section, you will
NOT see him again.
If you enjoy driving the Veyron, this race is a good challenge! Just keep in mind that you need to run a clean lap, stay off the grass, keep all 4 (at least 2) wheels on the ground, and to take the lead early! The AI cars seem to be very timid about passing you and are fairly easy to keep blocked until straightaway sections where you can put on some distance.
Once you put in all this effort to win the All Stars race, you get presented with the usual victory pittance, and crappy Amuse Carbon R (R34) '04 that has never left my garage!
Hopefully someone out there will find my experiences useful.
ALTERNATIVELY
If you have the 600,000ish credits to play with, and don't feel like puttering around with B Spec Bob long enough to win the Furai you could buy the Corvette ZR1 (C6) '09, race mod it, and buy all the upgrades for it. After some playing around with that, a good tuning (and remembering to put the bloody racing slicks on) I was able to post faster lap times on Indianapolis than I could with the Furai (was running consistent 42.6 second laps, no drafting at all). It's viable to run everything using the ZR1 including all stars, but it's more credit intensive than time intensive (leveling up your B Spec driver). I still highly suggest getting the Furai just cause it's a very fun drive, and also someone posted that they were able to complete the All Stars championship using a fully tuned Furai.
For those that want to know the tune I used for my ZR1, it's as follows:
Corvette ZR1 (C6) '09 tuning courtesy of Sail IC, from this thread:
https://www.gtplanet.net/forum/showthread.php?t=136110
Downforce: 35/53 (I set rear downforce to max)
Transm.: 360 km/h (380 at Nordschliefe)
Initial torque: 10
Acc sens: 20
Brake sens: 10
Ride h: 5/5 (35/35 at Nordschleife)
Spring: 16.6/14.4
Damp ext 9/7
Damp comp: 9/7
Anti RB: 3/5
Camber: 2/1.5
Toe: 0/-0.30
Brakes: 5/7 (I set rear brake to 6 for more stability)