How many miles on the engine?
Have you performed a WOT cylinder compression test? If yes, readings?

Some of the usual places for parts:
https://egge.com/
https://www.kanter.com/
http://www.rockauto.com

Tim posted this back in 2017. Prices will have increased since then:
1960 235. Good block, good head, bunch of new parts and machine work. They want $3220.83 for parts and machine work, including new cam, new lifters, new pistons and rings, new bearings all around, new valves and guides, plus boring the block, turning the crank, re-facing the flywheel, new seals, and a gasket set. Whew! more than I had expected, but I should end up with a great engine.

Food for thought:
If engine has been idle for a long time, the rings may be stuck. Search for stuck rings.
You can try running a different weight engine oil to raise oil pressure.
If you only drive the truck a few times a year, is that enough to warrant a rebuild?

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."