So I should avoid looking at Camaros with a TKO 5-speed? I know the 6-speed was used on Vipers, whats been used on later model Corvettes and Camaros? My 99 Vette had a 6-speed, but it was a lease and I just had fun driving it. Any 60's era Camaro I'd be looking at is going to have a built engine of sorts... you never know exactly what unless there's a build sheet. And most likely they will have a Muncie or BW 4-speed, and quite a few have TKO's. I know Richmond makes gears... I have seen some cars advertised as 5-Speed Richmond. What does that mean? The cars with Tremec 6-speeds are not all over the place although I have seen a couple. One was sold by time I called the guy LOL, the other one might still be around but its behind a built 383 SB. I do know that the Muncie trans will stand up to a lot of abuse, but honestly those days are over for me. I could easily live with a Muncie, the nice thing about a 5/6 speed is that they might have some overdrive aspect that allows more flexibility with rear end ratios.
I had a Muncie M-22 4 speed in my car for years - never had an issue with it and it held up very well to my abuse. I had a 450hp SBC and 12 bolt posi with 4:10 ratio in the back. That was a very fun car to drive around town, but on the highways it was just plain brutal. Loving that rear end ratio so much, I went with the 4:11 in my current build and fronted that with a T-56 Magnum(Tremec 6 speed) and a 572 BBC. I am hoping that this will be a killer setup. That being said, things that could change your consideration for a 4,5,or 6 speed car would be how the car you are looking at is set up.
If you have moderately built powerplant with 3:08 gears in the rear - a 4 speed would suffice. Once you get into the 3:55 or 3:73 and up ratios, you will want to consider at minimum a 5 speed.
A scenerio like mine above I worked out all my options and one other thing that came into play was the torque rating on my motor. Other than getting a custom built trans, the only one that that complied with my torque rating was the t-56 magnum.
Richmond does make transmissions as well although I have not heard anything good or bad about them.
Good luck in your searches