SM420 vs T 5 may depend more on how far you intend to drive the truck away from home base. If you never plan on driving it more than 50 miles from the house very often it probably is more prudent to just go with the SM420 or find a 47 or earlier factory floor shift 3 speed and the small pattern bellhousing to go with it.
If you plan to range further from home on a regular basis the T-5 will make things a lot nicer for the engine. I'm thinking that if you had the S-10 drive shaft to go with the trans it wouldn't be that hard to fab up a front drive shaft to mate up to the existing rear half.
You would have to figure out the carrier bearing setup but that might be the hard part.
it would cost probably 100 bucks with shipping but a Jeep 10 spline input shaft simplifies the clutch thing and lets you run a regular Chevy 10 spline clutch disk rather than hunting for a disk that matches the S-10 spline that is the right diameter. I think I got the last Astro Van clutch disk that Rock Auto had . Those things are seriously hard to find now.
Here is a how to on the Jeep input shaft swap
https://lugnutz65chevystepside.weebl...ter-plate.html