1974 D300 360 fresh rebuild will not start

Being the engine has not been started yet, did you prime the oil system with a drill by pulling the distributor out ?

By priming the oil system and having someone slowly crank over the engine by hand (with the spark plugs out . . easier) you are filling all of the internal oil passages and you are "Pumping Up the Lifters" at the same time. But need to rotate the crank and cam to fill the lifters.

Guessing the lifters are not pumped up yet only opening the valves partially on your compression tests.

More to come yet after you get back to us on the above questions.

Getting close to getting it fired up pretty soon here.