I'd look for loose shifter rods or a loose shifter.
When you say "hard to shift", do you mean that it slips out of second easily, then travels through the neutral gate, then jams (or sticks - or whatever you feel) going into third? Or is it something else?
When your aren't trying to shift faster, does it feel different, or because it is moving slower, the sensation is different (and less apparent)?
If the car isn't running, with the clutch pedal pressed, can you feel a difference between a slow and easy shift, and a faster/harder shift?
This is a new TKO 600 and I didn't use a kit from Keisler or Classicchevy5speed. The crossmember brace is stock and I had it modified a bit and I think it's too high. Could this be the problem? Do gears not want to shift easily if the drive line isn't quite correct? I should lower it about an inch shouldn't I?
To answer... it's only when I get on it hard that it doesn't go into 3rd easily. I have to jam it in there or even shift it very carefully.
Also, when I turn hard around a corner the driveshaft rubs the tunnel a bit and this is why I think I need to lower the crossmember brace a little.
new tkos are hard to get into third when its still new
keisler says that after about 500 miles of "break in" period it should shift like butter
i read it somewhere on this site - http://www.keislerauto.com/gm/transmissions/corvette_68-82.asp
my friend had the same exact problem on his firebird, it goes away after a while, but it sucks loosing a race and having to sound like a ricer by saying "i couldnt find third"