The transmission depends on what you want to do with the truck, and how much money you want to spend.
Have you decided on your intended use yet?
How fast do you ever plan to go / what type of roads will you travel?
The right transmission also depends on what engine and rear axle/gear ratio can use. They are all part of a package to be figured out together.
If you don't need overdrive, don't want to spend money, and like driving 40 or slower then an SM420 4 speed from an AD or later truck will bolt on to an in-line 53 series with the flywheel housing from a Gama Goat 3-53 engine available reasonably cheap.
If you like shifting, and don't mind an unsynchronized transmission then a Roadranger RTO-610 or RTO-6610 ten speed overdrive is a good choice, my personal choice.
If you want to spend money then an NV4500 from a newish Dodge or Chevy 3/4 or 1 ton pickup would be a good choice, 5 speed overdrive. an adapter will be required, but I believe they are available
If you want a really tough, cheep, and heavy transmission then a Spicer 3053a from a deuce and a half military truck gives you a 5 speed overdrive.
If you don't haul much you can use an automatic like a 4L80E.
An older Allison automatic would work, but no overdrive.
There are so many choices and all may be good ones depending one what you want.
I don't think we can recommend one over the other unless you give more info on your plans, goals, and likes / dislikes.
Grigg
