How hard is it to put a v8 into a 4cyl 1974 mustang?
You'll need another Mustang II with a V8 to be a donor.
Doing it piece by piece is going to end up costing more than a "real" Mustang with a V8.
The good news is that you should be able to find a 76-78 Mustang II V8 fairly cheap. You won't need to be concerned with body or interior. Just whether or not all the running gear is there. It doesn't even need to run really.(Because if I recall correctly the 70's Mustang 302 is only good for about 125 hp anyway....you're going to want more.)
You'd really have to have an exceptional 1974 Mustang II to start with to make it worthwhile. Since it didn't come with the 2.8l Cologne V6, I'm almost certain you do not have a Mach 1 and the Cobra sticker package didn't come out until later.
The good news is that it's a 1974. No catalyst!
All V8 Mustang IIs were originally automatic. I've seen Mustang IIs with later model (ie: Fox body) fuel injection and T5 manual transmission. I have no idea how much fabrication this took. Probably quite a bit and again would require a Fox body 'stang donor.