What to check after a few hundred miles on a rebuilt engine?

Bl1zzard I am reading your original post, if the car engine was rebuilt sometime back in 1984 and driven for some time so its past checking torque. If something major was gonna loosen up it would have done it already. What I was recommending was for a newly rebuilt engine.

Maybe it is as simple as a leaky valve cover gasket, or loose valve cover bolts.

Btw if you want to correct the fact that it cranks rather long after sitting a few days, ditch the stock mechanical fuel pump, put a block off plate, install an inline electric fuel pump designed to work with a carburetors lower pressure requirement, then switch the pump on, let it run for about 30 seconds before you pump the gas to set the choke and fire it up. Something like what's in the pic below

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