I've got a 78 280z in real life that i'm doing some work on.
in game I used the 240ZG (because the 78 280z-L is actually a US spec 280zx, diff. chassis)
mods:
racing exhaust
stage 1 clutch
stage 2 flywheel
racing trans (to get the gear ratios perfect)
sports suspension
sports chip
NA tune 1
S2 tires
Port and Polish
Engine balance
and I think that's it.
on my Z i'm rebuilding the engine right now, but when I finally finish with the car it'll look like this
Light weight flywheel (11lbs I believe)
Megasquirt ECU
ported intake mani/custom mani (not sure yet, this is why i did the port/polish on the GT4 version though. the manifold currently is 53% smaller than the head ports, i'm going to fix that problem my car has hah)
Internals are going to stay stock really, except a full engine balance.
I had to use the NA tune 1 on the GT4 version to compensate for the lower displacement of the 240zg, and also because my engine has a slightly higher compression due to the head being shaved.
It'll also have full twice pipe headers and exhaust, running through some small straight through mufflers (loud and nice sounding)
the reason for the S2 tires on the GT4 car, is due to the fact that stock they run 75/90/14's and my car currently has 70/225/14 of a higher quality. basically, the tires are freaking huge. they're the widest tire you can fit under the stock fenders. And they don't really like to burn much at all. the S2 tires were the closest I found to simulating the amount of tire burn my car gets. I think i'll have to upgrade the suspension though because i'm getting stiffer springs for my Z. the sport setup only lets you adjust the shocks.
power output in game after an oil change was about 212hp. this seems to be right around where my power output will be when i'm finished.
Here's a tip for people having trouble with gear ratios. go ahead and get the full customizable one. the Auto setting will skew the ratio range for each gear. you just have to play with it till you find the correct range where you can adjust for your gears. in my case i had to use an Auto setting of 20 (iirc) before I was able to get the exact settings I needed.