I for a Greenwood wing, but they don't make C3 stuff anymore
I have that same modification, and it must give a lot more downforce that the rounded original end. I do not think it is enough, however.
I tried to get most of the Greenwood kit from Greenwood, but they are not making anything for older than C4 anymore. I wanted everything except the main side effects, where sidepipes will eventually go.
Several of the big catalogs have some wings they offer that specifically work with the built in 80 style rear and they aren't monster blinger style ones you see on an otherwise bone stock ricer that couldn't hit 110 in a tornado. Ecklers, MAD, etc. offer these in their regular cataolgs and they look nice.
I do not know if these have been TESTED for aerodynamics in a wind tunnel like the Greenwood stuff was though.
The Greenwood people were nice to "talk" to, and apologized for not intending to make any more C3 kits. They told me the kits occasionally show up for sale or at a swap meet. Maybe if enough of us bugged them and did some sort of group buy or commitment they may change that - they certainly have the tooling still sitting around.
I occassionally meet a C3 freak with an 80 and a 600 cube mill in it. His has the same upwards tail as ours, but he said the rear end got so iffy over 160 he swapped to a damned solid rear non-IRS. Apparently the rear end wasn't lifting enough to lose control, but it was rising enough to allow the IRS rig to camber the wheels in at the bottom.
I was horrified at that - to me the IRS is what MAKES it a vette!
I noticed something else though. I assume yours, like mine, has the rear deck vents that were part of the astroair system in soild top cars before a certain year. My friends does not.
Whether those vents are functioning the way they were designed to or not they DO provide an enormous vent from the bottom of the vehicle to releive much built up pressure that is missed by front and side air dams. When I had mine off to be chromed I put some strings on pieces of tape down in the holes. I had a bud riding and I took it up to 120 or so and was amazed to hear the strings came straight up and bent into an "L" about 2-3 inches off the deck! The force of air coming from under the car was so much it ripped some of the strings OUT of the little strips of duct tape! My bud said the air right over the vent surfaces was so strongly moving up it was like an air hose.
I still want a small wing back there, but I would like to have one that had been actually tested in a tunnel - and NOT some horrible rivetted aluminum monster that looks like it came off of a metallica-world sopwith camel on acid!
Those monster wings also can rob a LOT of effective hp from the added drag if they aren't designed right.
But that's my opinion, take it for what its worth! 
agreed
