Just to followup on the TKO 600 vs T56 - as already stated, the T56 has both overdrive gears - 0.80 and 0.63. The reason I wanted the T56 in my cobra was so that I could track it and have a usable 5th gear plus have a easy cruising gear on the street.

The TKO 600 comes in two flavors - the road race version which has the 0.82 5th gear which is great for the track, but not great for cruising at 75-80. The street version of the TKO 600 has a 0.64 5th gear which is great for cruising but useless at the track - also cruising around 55-60 mph can sometimes be a nuisance with the 0.64 OD - it's a big drop from 4th to 5th in rpm with the street version. It also depends to some extent what gear you have in the rear diff - I have 3.55 which is pretty common.

The downside to the T56 is it's a huge transmission - it's a beast to install. You must have IRS to start with as the driveshaft is tiny - the center cylindrical section is about 2" long. You will probably have to modify the tubing in the trans tunnel to get it fit - it's just a little too wide, plus you will have to come up with a custom made transmission mount as the standard A trans mount won't work.

The upsides to the T56, in addition to the two OD gears are that it shifts like butter - much smoother than the TKO's that I've had especially shifting into 1st gear. It also has a couple of other benefits such as a low speed reverse lock out solenoid - you can't accidentally shift into reverse until the car is at a stop although that is not usually a huge issue. It also has an easy midshift conversion - the TKO is a pain to convert to midshift involving removing the tail shaft etc. With the T56 you don't have to do that and deal with roll pins etc. You just have to install the shifter and you're done.

I've had both versions of the TKO 600 in my car, and the T56 is a much better tranny, but it does take a bit of work to massage it into place.