Yeah, a 4.6 would require major surgery to your car. Losing the fender wells and reinforcing the subframe at a minimum.
I had one given to me back when I owned a different 79 with the intention of doing that swap, but unless you are a very good welder/fabricator or know one who will also work cheap it probably isn't worth it.
If you happen to have those skills, have fun. It could be an awesome swap. Although most here would say you have ruined the car since this falls into the "Can't reverse the mod" so many feel is important. It has been done with a Mustang II a few times, but not exactly the same engine compartment as our Pintos.
Russ