What would cause engine to jam while cranking??

What RRR said . ∆ ∆ ∆ . .
I generally put #6 rockers in overlap, that means #1 is firing, timing mark should be at zero.
Drop shaft/distributor to have rotor pointing at #1 wire on cap.
Give it lotsa initial timing, - 12 - 25*s, an engine without lotsa timing will generate excess heat, fill float bowls, shot of gas down the carb.
Vroom, check for oil pressure, look underneath for leaks.
Set timing, check temp .
Good luck.