I have an engine out of a 1994 Mustang GT. I've been researching to see if the engine would fit in a 1971 Mustang body but haven't been able to find anything. Any tips?

It's not a matter of fitment, it's just a matter of adapting to run outside it's native model. (I would check for clearance issues though with the trans tunnel.) Go join a Mustang club or forum for guidelines on making such a swap, as it's been done by others. You can also check out speed catalogs like Summit, Jegs, and other (just Google "speed equipment" for more). You may find engine mount kits for such a swap in there, universal cross members, etc. You can also find engine wire harness kits with aftermarket engine computers so you can run the modern motor in an older chassis, stripped of all the unnecessary things like security. You'll have to make some other modifications, such as to suspension (to accommodate the lighter motor), exhaust, and plumbing. (by plumbing I mean adopting an electric fuel pump, modifying brake booster and other vacuum actuated features for change in engine vacuum). Lastly you would likely need to redo the dash gauges, since none of those would work with newer engines sending units. Again, plenty of kits out there to accommodate all these changes.