[AnthonyG]

Im not an expert on Flattys for sure, done the normal pull plugs replace & tune it up. I e had many Mid ‘60’s GM engines apart & think after building startup should happen right after rebuild to seat everything, more time unstarted amplifies w Unstarted new rebuild low compression is possible until seating internals. That one left the garage so try this before disassembly. Drain a quart of oil & replace w STP poured equally in each spark plug holes & turn the engine over letting STP coat cylinders & rings. Lightly Redress plugs w point file reset & Replace plugs, prime carb w fresh gas (have fire extinguisher handy) no ethanol if possible & see if she fires up. If so if all sounds good don’t shut it down & let run for 30 or so minutes. STP should give compression a boost needed to more easily start as long as if as stated all setup right when built.