Welcome to the Action Center, I'm sure you will find a lot of usefull info here.
As for your question about the tires. It realy depends on what size tire/wheel combo you go with. The front 275/40's, and 315/35's used on my GS should fit under the wheel wells of your 81, but you would need massive spacers which would make the tires stick out a few inches from the fenders. The tires used on the early C4's where smaller, 245/40s' all the way around I beleve. They shoud fit your car, but again you will need spacers, and perhaps an adapter to match the bolt pattern on the new wheels. The same would go for putting C5 wheels and tires on your 'Vette. As far as 4th Gen F-bodies go, my '99 V-6 Camaro came with 16in wheels, and I beleve 245/35 tires, the Z-28 and SS models had 18 inch wheels but I dont know about the size of the tires. Though the same would hold true with these tires as with the C4 or C5 tires. Also keep in mind that changing the size of the rear tires has the effect of changing the final gear ratio of your car too. A larger diamater wheel will lower the ratio, thus giving you a higher top speed, and a smaller tire will raise the ratio giving you faster acceleration. It will also throw off your speedo.
Check out www.tirerack.com they have a massive amount of tires and wheels, and are very knowlageable about this kind of thing. You can also check out Eckler's and see if they have the spacers to do what you want. To order a catalog, or to talk to someone the phone number is 800-327-4868.
Good luck with your switch, and I must agree with mthom, C4, C5, or 4th Gen F-body wheels and tires on a shark would look great.
Grand Sport #328
Borla Exhaust, K&N