Types of 3550s

Tremec improved the original 3550 over the years and increased the torque rating with each variation:

3550 = 350lbs-ft
3550 II = 375lbs-ft
3550 TKO = 425lbs-ft
3550 TKO II = 475lbs-ft

There are several companies out there that sell rebuild and upgrade kits that increase the torque ratings even higher. So, if you find a screaming deal on one of these earlier Tremecs, don't walk away from it. You can possibly build a trans that will be just as strong as a new TKO600 while saving a buck or two by doing the work yourself.