I have experience with the ZF 6-speed, the T-56, the Richmond 6-speed and I prefer the 4+3 (for reasons outlined in my recent "4+3 question" post)...
My brother transplanted a brand new Richmond 6-speed into his third-gen Trans Am... lots of problems getting the clutch sorted out (vibration, wear), and the tranny was pulled back out 3 times and exchanged by Richmond once (work was done by a tranny shop experienced with high performance stuff)... in the end, everything was sorted out and it worked fine... but watch out: this is a very heavy-duty tranny built for racing applications. It has straight-cut gears and it is very VERY noisy! Not a problem on the track where your primary concern is reliability and performance, but I could never live with it on the street. It shifted very hard for a few thousand miles and then gradually loosened up. (He tried two different shifters too.) My brother would strongly recommend something like the T56 if you decide to go with a six speed... advice from a guy who ended up spending over 5 grand on his conversion.
Remember too that those six speeds (except for the Richmond) have two overdrives... which makes them 4-speeds plus overdrive in my opinion. My wife has a 2001 Camaro SS with T56 and Hurst Shifter... don't get me wrong, the tranny is sweet... but I still prefer the 4+3.
Check out
www.gearzone.net for more info. His opinion is that the pre-86 units had problems but can be rebuilt to be reliable, and the 86 and up units are reliable if the fluid is changed regularly...
He also says that the 4+3 costs less than half as much to rebuild as the ZF six speed... and GM doesn't support either one of these trannys anymore anyway, so that's a wash.
There's a lot of negative opinion out there about the Doug Nash, and I understand it... a lot of people have been burned and they are entitled to a strong opinion. But count mine as a strong yes vote. I have an 87 too, and my OD unit was rebuilt under warranty at 27,000 miles. It was then run without a fluid change until nearly 90,000 miles... which is a big no-no, but it still works fine. It now has 105,000 miles and I have changed the fluid twice since 90k just to be sure, and I will change it every 15k to keep it healthy.
