I'm not familiar with the MSD dists. but the factory HEI should fit. I have put them in various small block engines of different years & they always fit as long as there is firewall clearance. I understand the same dist fits big block also. If the tangs are in the slots it should bottom out. If not something is wrong with the dist.
There are no tangs, the oil pump drive shaft is entirely different.
The Oil pump drive coupler on the bottom of the original distributor is a rectangular deal where as the HEI has the "flat scre drive blade" type thing to engage the tangs of the typical small block oil pump drive shaft. Is there a way to attach pics?
