Lets start with one thing. The most successful C4 is probably the 89 Z51 cars. The had the best suspension package, lowest weight, best gearing, of any of the line. But it was only rated at 245hp, but they did win BSP more than a few times over other C4's....but that was about 15 years ago or so. I'd want the extra RPM's and HP out of the LT1 personally.
Within the LT1 years 92-96 the Z07 package was available from 92-95 and the 96 the Z51 was made available again.
The 96 Z51 package is crap. The spring rates are WAY too soft for anything IMHO other than highway cruising. The LT1 was only available with the automatic transmission. If you wanted a manual in 1996 you got the LT4 which is not eligible for STU. Not sure why but its not.
The only real difference between the 92-95 Z07 and earlier Z51 packages is that it came with the adjustable FX3 Bilstien shocks and it had a lower rate front spring rate. 88-90 Z51 cars had the 115N/mm front spring/57.2 N/mm rear spring 30S/24S spring/bar combo. 92-95 had the 90.1 N/mm front spring/57.2N/mm rear spring 30S/24S spring/bar combo.
Your correct the for the LT1 6 speed cars the ONLY rear end ratio is 3:45.
If I was buying a C4 like you describe I would get a 93-95 Z07 coupe with as little options on the car as possible. My basically fully loaded 40th Anniversary coupe in B Street trim weighs in at 3300lbs. I might be able to get the car down to 3200lbs in STU trim. The 92's had 10ft-lbs less torque than the 93's due to a camshaft change.
Yeah the car is not competitive in B Street. Maybe at the local level its still a fairly quick ride but you'll get owned by C5's and such as long as they have talented drivers. I enjoy running mine and its quick enough to be competitive at the local level. I think it stands a better chance in STU for the following reasons:
1. The car is NARROW. The C5 is something like 73 inches wide. The 350Z (won this year) is about 71.5. The C4 is 70.7.
2. STUPID short wheelbase compared to the C5 and 350Z. Those are both over 100 inches. C4 is 96 inches.
These are NOT small things in the world of auto-x. You can fit in smaller spaces and change direction quicker at least in theory.
The C4 has more torque than the 350Z. Better gearing than the C5. Power to weight is about the same ratio on the C4 and 350Z. Better weight balance than the 350Z.
The C5 makes a lot more power and is lighter (not sure this really matters on street tires?). The folks over at Corvette forum noticed in some video comparisons from the winning 350Z to their C5's, how much less wheel the Z needed and how much more the car was "on throttle".
Your right on parts being unobtainium. Clutch parts for the ZF gearbox are particularly troublesome......if you want quality. Ask me how I know. New Old Stock is all you find but its all gone for the most part. I freshed my 93 with probably the last NOS suspension stuff around.
The best part of STU is you can fix the C4's short comings. Mostly lack of front end camber with offset lower control arm bushings and you can get poly for everything else (do not put poly in the 4 trailing links, they bind, you can get these new with rubber bushings, however).
I've done a lot of reasearch on C4's and learned a ton. They have another fundamental weakness that cannot be fixed in STU. The rear suspension geometry is such that if you get much more roll stiffness than the factory Z51 NYU rear spring (57.2N/mm) with the 24mm Solid rear bar the handling goes to crap. I found MOST if not all C4's from the past in BSP would run smaller rear sway bars 20mm or 19mm solid with a stiffer 84 Z51 spring or some sort of adjustable spring/bar combination. The point is you cannot go much stiffer than the stock 85-95 Z51/Z07 rear spring/sway combination.
Those same BSP cars run lots of camber/caster up front (so more mechanical grip) with very stiff (1200lbs/inch in some cases) springs.***Side note: the Factory 88-91 front 115N/mm spring is about 650 lbs/inch***. I don't know how "balanced" these cars where but I'm sensitive to these things and feel a new approach is needed for STU C4's. If the rear is limited to basically what you have from the factory then why ruin good handling balance with at STUPID stiff front spring and the interested roll stiffness.
I'm going to play with my 93 and develop it into a STU car, slowly. I'm starting with taking my B Street setup and adding the offset lower control arm bushings to get the camber. Drive it, see how it acts. If it is too loose, and I cannot adjust with front bar, then I will put a 88-91 Z51 front spring on the car and repeat the testing. The super stiff, soft rear spring approach I feel is wrong and will ruin the cars balance.
I had a friend drive my car (B Street) and a BSP C4 back to back. He RAVED about how much better the balance was on my car. It didn't have the all out grip or grunt but it had a MUCH better balance. He can drive to, granted its karts, but he can drive nonetheless.
BTW, Koni makes a really nice sport shock that just happens to be matched to the Z51/Z07 spring rates. Oh and they are fairly cheap. Work great too!
Hope this helps. They are fun cars, I really enjoy auto-x'ing mine. Its been to a few track days too. Dare to be different.