1974 D300 360 fresh rebuild will not start

That spark snaping sound, sounds like the coil lead jumping to ground or jumping between cylinders at the top of the distributor cap.

Have your wife turn it over at night with the hood open and see if you can see where that loud spark is jumping at in the dark, this could be why it does not start.

So it looks like it is down to faulty spark to figure out and it will run.

Too bad you don't have access to a simple small block points distributor, one hot wire to the coil and it is running. Distributor lead to negative side of the coil.

When running need to make sure it is full of coolant and oil and let it run for a couple hours at a fast idle so that your new rings start to break into those smooth cylinder walls, that is the last bit why your compression is low . . rings need to get broke in.

Keep an eye on the engine temp as it is running, don't want it overheating on you at first run up.