Ray, it's easier than that... When #1 piston is up, #4 is down.
Put your timing mark on TDC and don't move the crank. Stick the rod down #1 spark plug hole and mark it. Pull #4 plug and stick the rod down the hole and mark it.
Measure between your marks. The 352 stroke measures 3-1/2" between your marks. The 390/427 crank measures just over 3-3/4". So, if you measure more than 3.5" stroke, it's a 390.