for lack of better terms big block or small block (Ya I know those are chebby terms).

351w are also more plentiful if you end up popping it. (most of the 80's some of the 90's in in f-series - bronco's - vans)
351m can be turned back into a 400 if so desired.

I think you might be able to find EFI for the 351w where as you would have to either hunt for aftermarket 351m EFI or rig somthing up. Of course that is if you wanted to go that route. I also think 79 is the last year for the 351m.

The 2 motors require differant motor mounts and trannies so once you pick it you'r kinda stuck. Unless you don't mind another project to swap it over.