First off, I just wanted to say thank you to all of the people here that have posted different solutions for conquering this race. It's definitely the most difficult B-Spec challenge I've run into on GT5 yet.
I don't have a 2J or a R8 Race Car so I had to make do. After trying a bunch of different cars I finally won with the C60 Hybride-Judd Race Car '05 tonight.
The only settings I changed were:
Ballast to 1000kg.
Power limiter to 840hp.
I added the chassis reinforcement modifications. I had the stage 3 turbo kit purchased for the car too but realized it actually puts out more power at 700pp without any power upgrades.
I used two level 31 B-Spec drivers. Started the more aggressive of the two out on RH tires. Pitted after lap 2, switched drivers to the calmer driver, RI tires. Came out of the pits in 10th place. After passing much of the pack in the rain and everybody pitting after the 3rd lap I was in 2nd behind the other C60 Hybride going into lap 4. My 2nd driver did great at just tailing him and not losing ground while we left 3rd place and back far behind. I pitted after the 7th lap, switched back to the more aggressive driver, fresh set of intermediates and came back out in 4th place. My driver pulled into 2nd towards the end of the 9th lap and blew by the 1st place C60 while he was still in the pits. I slowed him back down since the 2nd set of IR tires were starting to wear thin and didn't want him to make a stupid mistake. The 2nd place C60 was gaining on him the whole 10th lap but he stayed smooth and finished 5+ seconds in the lead with a total time of 40:37.
Again, this wouldn't have been possible for me had I not read through this thread and followed in the footsteps of those who had beaten it already. Thank you again.