58 283 truck engine - won't turn past 1/4

[bhap]

I'm looking at a 283 truck engine that has been sitting inside for years. History is unknown and when we try to turn it over, it will turn by hand, but no more than 1/4 turn then "thunk". Back the other way, same thing - 1/4 turn then thunk and a hard stop. While doing this we looked down the distributor hole and the slot on the shaft that drives distributor is not moving. Any guesses at what this might be? Also, is a 58 "M" code 165 hp, truck motor an ok block to rebuild? This will be going into my 62 C10. Once we pull the pan we'll see what's wrong so I'm more interested in knowing if this early block is a good starting point to build a stock-ish 283. Stamp on pad is F0404M. I don't have casting number.