my 50 3/4 ton is a 4 speed. i don't know how much the gearing with a 3 speed in conjunction with the different rear end gearing helps, but mine definitely runs out of steam at 50mph. if you're not loading it much anymore i think i'd look at the rear end swap to accommodate the tire size/road speed you're looking for.
maybe you already know this but you may want to check patrick's for some gearing options for the rear.
make sure your steering and brakes are up to the task before you test the speed limit...
