235's aren't scarce at all. There are still plenty of overhaul parts available, and good-running used engines can be found with very little effort. Just about any 235 from mid-1955 to 1961, car or truck, would be a bolt-in swap. Check Craigslist and/or Ebay, plus any local wrecking yards that might have older vehicles. Another option would be to find a 261 engine from a medium-duty truck from the late 50's/early 60's. The appearance of the two engines is almost identical, but they've got considerably more torque than the 235.
The 62-up engines, either 6 cylinder or V-8, have a different bellhousing bolt pattern and somewhat different motor mounts, so quite a bit of adapting is required to do an upgrade to a later-model engine.
Jerry
