Originally posted by panel boy:
IF the '79' has it's original motor;the 350 in trucks will have an 8 inch harmonic balancer. the 400 has 3 freeze plugs in the side of the block while all other small blocks have two. you can also look up the casting number at the back.
The freeze plug thing is not a hard and fast rule. There are some exceptions...although I can't remember them off the top of my head. It's true in most cases, tho.
However, the dead giveaway (or good news, depending on how you look at it) is the asymmetrical harmonic balancer on the 400. They're externally balanced motors so the balancer and flexplate/flywheel have the balancing weights rather than the crankshaft.
