Any 235 or 261 can be swapped into your 50 pickup but the 55-1st and earlier will be the most direct. Some car engines will have side motor mounts and the front cover plate where it turns out horizontal may not be drilled for the front center mount. The 55 car and 55-2nd truck and later engines have the low-style water pump: this is not too big of a deal. Adapter plates to use a 216 pump, and short-shaft pumps are available. If your truck is still 6 volt you will want to retain your starter, flywheel, bellhousing, clutch, generator, etc. The starter and ring gear on flywheel must match. And they made a change in the bellhousing; how it mounts on the crossmember in 54.
How was the engine running prior to this episode? Was it smoking or using too much oil? Running rough or lacked power? You might consider replacing the timing gear(s) and putting it back in service. That one valve striking the piston does need to be corrected... The cam may be stuck in a bad position holding that valve open. Any bent pushrods? Have you pulled the head to inspect cylinders, pistons and valves? Your motor might not be too bad. Good luck!!
