Best transmission can be defined several different ways. One might consider the best transmission the cheapest one. Another might consider the best transmission to be the one with the lowest first gear. Others might call the best transmission the heaviest duty transmission, It really depends on what you are really asking.
From your question, I'm assuming you feel a t5 will be a bit to light duty behind your stovebolt 235 in a 55.2, and you are looking for a stronger alternative. I also assume you are desiring an OD to go with your 3.90 factory gears.
I believe a T5 would be more than adequate in a 1/2 ton with a 235, but if you want more options...
A833 (MY6) New Process 4 speed is tough as nails, and almost a bolt in swap. Fourth gear is 0.73:1, or a 27% OD. Not good enough for you? Try looking at a NV4500 5 speed. This 200lb tranmission is found in gm and dodge 3/4 ton pickups. It is very heavy and has the advantage of having a low first gear and a 0.73:1 5th gear. Of course you would need an adapter plate to bolt it to the stovebolt bellhousing, and the adapter plates that are available for the task require extra machining.
The t5 is probably the easiest and cheapest swap. Although I would never put one in a 3/4 ton or larger pickup, or a 1/2 ton with a v8, it should be more than adequate for a 235.
John
