I just put a set of OEM C6 wheels on the Z. I bought some 1.25 spacers for the rears. It looks great (sits higher though) but I lost alot of straight line traction. Could this be related to tire brand (continental sport contact2), compound, psi, shorter sidewall? How can I remedy this issue? I put stock vette size tires (285/35/19) thinking wider would be better.
My next question is will 305/30/19s fit with my spacers? I heard some pretty good reviews from the vette guys about nitto invos in this size. Sorry for all of the questions, Thanks in advance.

 

Large diameter wheels, with tiny sidewalls are death to straight line traction. The sidewall has no "give", and will break loose. Might get away with it on a Corvette with a fully independant rear suspension, but put them on a live/solid axle f-Body and they will probably destroy cornering traction as well. Serious drag racers run 15" wheels at the track..... for a reason.

 

Well I have 30 series 18s with BF Goodrich KDW IIs, and I don't have huge traction issues... Actually 1000X better than the GSCs the car came with...

While I understand that low profile does come with some traction loss, I would lean towards those particular tires being the issue. Sorry to say Tirerack has pretty mediocre ratings on those tires:
Max Performance Summer Tire Survey

Maybe swap out the rears for a different tire
BFG KWD IIs
Or
Others have had good luck with: Eagle F1 GS-3 (If you can find them)

 

MT drag radials are your best friend

 

Yeah I have a set of et street radials that im going to throw on my old salad shooters which will definitely help. Ive also got 4.10s (not installed) which are going to murder my traction.