Hi Draco,
Personally I like the T56. It has a good spread of ratios. The ratios in mine are as follows:
1st 2.66
2nd 1.78
3rd 1.30
4th 1.00
5th 0.74
6th 0.50
The standard TR3550 ratios are:
1st 3.27
2nd 1.98
3rd 1.34
4th 1.00
5th 0.68
The taller 1st and 2nd in the T56 makes for a much closer ratio box without the huge gap between 1st & 2nd that the TR3550 has. I plan to run a rear end ratio around 3.9 or 4.11 so the lower gears should provide some great performance but I'll still have a good cruising gear with the 0.50 overdrive in 6th.
The T56 has the same or better torque rating as the TKOII depending on which model. They range from 330lb/ft to 550lb/ft where the 3550s range from 350lb/ft to 475lb/ft. They are standard fitment in the V10 Viper and 007s new V12 Aston Martin so I figure they must be tough.
I'm running a Gen 3 Chev LS1. These motors make great HP numbers but their torque is higher up in the rev range. I figure the closer ratio box will help get the best out of it.
Perry is right about the length of these boxes, very long. The driveshaft in my car will work out to be about 15" long. Other things to think about is that the standard shifter location works out to be a long way back (there are ways around this). Also it only supports an electronic speedo. The standard integrated bellhousing may also be a problem unless you find a Mustang box. Sallee Chev sell a T56 that bolts up to a normal aftermarket bellhousing or scatter shield using an adaptor plate on the front of the box.
Sallee has a good site with pics and dimensions on their T56
http://www.sallee-chevrolet.com/BorgWarner/t56pics.html
And the T56 page at tremec which gives all the part numbers an ratios for the different models.
http://www.ttcautomotive.com/English/products/T-56.asp
Cheers
