I believe there is plenty written about the fact that the FRC or (Billy Bob - Its project name) was originally conceived as the low buck stripper Corvette. It was going to be the roll up window, cloth interior, boy racer car.
There has also been alot of print about the fact that GM had to use the FRC because of weight to meet the LEV specs with the LS6. That is also why it has been stated that the LS6 will not be offered in the coupes or verts.
The story about the weight may be a load of hooey, but that has always been the official story.
As a side note on the LS1, and LS! development. GM had one of their engineering mules here in Houston in 93/94. It had the GENIII (LS1) engine in it, and it was on loan to Johnny Klevenhagen (the then Sherriff of Harris County). Harris County had ALOT of Camaro interceptors at that time. In addition Klevenhagen had connections at GM, and was about to go into partners on a Chevrolet Dealership. I saw this car at HRP during one of the Nationals. In talking to the folks, they claimed over 400HP then. They wouldn't open the hood, so I stuck my head under the car and got a look at what I could. They even made a pass in the car, but turned off the clocks so no one could see what the car would do. Now, if you look at the mods GM made from '01 to '02 they are numerous. GM also can't do mods like we can and get away with it. Just like the camshaft and valvetrain upgrades, they can't just throw those on and hope they work. They have to build motors, drive them, and then flog them till they come apart, and then find out why. They don't want folks like me coming in with a broke pile of parts under warranty going "This is a pile of garbage". Think about this, the LS6 is just the LS1 the way it should have been. It incorporated things that they found as time has gone on and needed fixing like improvements to cylinder heads and improvements in the windage control, etc... But come on, everyone knows GM always hold back a little. They don't put all their cards out on the table at once and they never have. Just look at any car Camaro/Firebird/Corvette. Why is it that the first few model years always have hideous wheels, tires, and ground effects. Look at the last 2 Generations of cars. Why couldn't an 82 Camaro look like an 89 IROC. Because it allowed GM to keep making small changes and keep attracting buyers. It is the same on the C5.
To meet the stricter LEV specs that is why the Z has some of the weight saving features, a few of which made it into other cars.
Lighter wheel/tire package
No Spare
Thinner glass
Ti Exhaust
Some of the other doo-dads from other cars like adjustible ride control, and tire pressure monitoring also added unneeded weight and were dropped.
As I remeber it the Corvette team was given $10 to spend for every pound they could shave off the car.
As for the frame on the C4. There was a very good article on one of the the Callaway race cars back in 1989 (I think). It used a regular c4 coupe, I know some folks added the convertible x brace which just bolts to the bottom of the car (but I don't know if this car did) But, what I do remember about the car was this, they ground all the spot welds on the car and seam welded every weld joint in the car along with putting a cage in the car. This is where they picked up the rigidity.
I could be 100% wrong on this, but it has always been my understanding that in the c4 and c5 lines the frames weren't different, but that there was possibly some additional bracing that was added on the assembly line (like of the c4 verts where they just bolted an x brace to the floor).
If someone has more definitve answers on some of these topics please jump in.