The point is the MB trans shifter mechanism is garbage, and there isn't really anything that can be done about it. It's got luke-warm gear ratios. It was made for low end luxury cars. Why limit yourself to one box, that's rare as all hell, and sucks to begin with, and itself is overpriced? Odds are the driveshaft and shifter need modifying anyway, so that makes the ridiculous setup of those boxes more trouble than they're worth. The whole point of an inline transmission is to avoid rods and cables!

Lots of 6-speeds are out there. Most likely, just picking up that T56 is going to come with Mustang, Camaro, or maybe Viper gears, 2 of which are basically useless except for holding at low RPMs on the freeway. Even the Tremec website shows all configurations have direct 4th.