Hi kevin,
If the engine spins freely without spark plugs, then it sounds like the added compression, with spark plugs in, is too much for it. I'd squirt some engine oil down the spark plug holes & spin it with the starter motor for a minute & see if the spin speed increases, as the oil does its thing. It should spin very freely, at speed, without spark plugs.
Remember every drop of oil has drained to the sump in a motor that has been sitting for years.
If it has been sitting for some time, there could be other issues like stuck valves & the like. Clip spark plugs back on the leads, with spark plugs out, but touching the block or head, spin the motor again, & see if you get sparks across the spark plugs. Removing the rocker cover & observing the tappets, will show you whether you have any stuck valves.
Cheers Banjo