Tannin,
It sounds like it is time to take off the oil pan- you shouldn't hear any clunks or thuds when you turn the engine by hand. Before you get into removing the oil pan, see if you can push/pull the flywheel back and forth, fore and aft. If it moves perceptibly, there is a thrust bearing matter to resolve. The thrust bearing is the third main bearing from the front. You can skip the flywheel check if you want to, and take the pan off- you will have to take the pan off anyway to see what is going on.
The 235 engine might be a better alternative to the 216, mainly because the 235 of that year has a full-pressure oiling system, insert bearings for the connecting rods, and aluminum pistons, all of which make the 235 easier and cheaper to rebuild than the 216. The 216 is a nice engine, but it has a splash-feed oiling system for the connecting rods, which means troughs and pipes in the oil pan to direct oil to dippers/scoops on the connecting rods to pick up oil. Also, the connecting rod bearings are babbited bearings, which means the babbit metal (bearing metal) s fused (soldered, so to speak) to the rod. That can be re-done, or you might be able to find babbited rods ready to go, but either they have to be adjusted individually with a time-consuming method, using shims provided with the rods- unless somebody has improved on the technique. I think there are insert-bearing conversions for babbited rods, but I don't know anything about them. Probably someone else here can add to that point.
Also, if the 235 is a passenger-car engine, the front mounts are on the side of the engine, rather than on the front plate. The right-side mounting bracket is near the fuel pump, with three holes in a V pattern (if the bracket is missing from your engine, or if it is a truck engine). Left side is similar. I don't remember if the front plate has the holes for front-mounting as your truck has, but if it is a truck engine, it will have the mount holes; probably someone else here can answer that question. In favor of interchangeability, the '59 and later 235 blocks were made to fit into the older chassis, so the front plate might already have the proper holes for front mounts. It comes to mind that the '59 and later passenger car engines didn't have mount holes in the front plate, but that may be foggy memory...
Hope this helps.
Harvester
