The original MT bulkhead flat has a flat floor, and any coupler can be easily body mounted, although some filing of the end sill will be needed if the box is mounted directly to the floor. They also sit way too high. Yes, the body mount kit should work, but it will still sit too high. If that doesn't bother you, this is the easiest way to do it.
It may be possible to install low-bolster trucks, such as BLMA or ESM, but those weren't available when I lowered mine. If you go this route, you might have to grind away some of the underframe for flange clearance.
All I did was cut the ends of the underframe off, to clear the trucks, and built new bolsters. Two strips of styrene will do the job for a bolster, both 0.125 inch wide. Cut a piece of 0.060 to fit between the side sills, then a piece of 0.030 about 1/4 inch long to go in the center of that. Drill a 5/64 inch hole in the center, and reinstall the trucks, without the couplers. It's easier if you mark the location for the hole, before removing the underframe, then mark the edges of the new bolster relative to that, so you get the new hole at the same spot.
