Put a kit into it. Pretty easy includes all new bearings and needle bearings too and gaskets for about $125 on flea bay. You will need a press and some bearing separators from harbor freight. I've done about a dozen of the sm420, pretty straightforward. Syncros are still available but if it is shifting well wouldnt bother.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration