90-92 all the tpi's were speed density.The tpi's were changed every couple of years 85-86 are the same in 87 the intake bolt pattern was changed in 89 the cold start injector was removed and in 90 they went to speed density. 1990 was the last year for the IROC and the year they started Gluing and welding the rear body panels to make the car stiffer and quieter. The 350 was automatic only and the 305 was auto or manual.The 350 was hard top only so if you want t-tops you will have to get a 305. You can check out
www.thirdgen.org they have a lot of info on there and some are selling their cars.
I found mine local in the autotrader. There are some on ebay but I would search only the ones that are local.
This is what it looked like the day I brought it home.I paid $3400.00 for it.