While an EFI engine will probably just drop right in, you have a couple things that might be an issue. First off, the fuel pump boss is there but the slot is blocked off so you'll either have to go to a electric fuel pump, or cut it out to use the mechanical one.

Also if it is a real late EFI block, it may not have the Z bar connection at the back of the block. This is for the mechanical clutch setup, so if you don't have it don't worry about it.

Lastly you have to worry about the fast burn head, which is what all EFI engines had since beginning production. This one isn't so bad to deal with, it just requires adjusting or "curving" the distributor to more precisely control just how fast the timing comes in. You may not even have to do this, but more than likely if you don't the motor will ping and not run correctly because of it.

IMO if it were me, I'd buy the EFI engine anyway just because it's possibly lower mileage, and none of the things I listed are a deal breaker they just need to be dealt with.