There is a sticky at the top of the general forum that outlines 4 to 8 swaps.
Basically, the only thing the same is the body.
To do it properly, you will need (among other things)
wiring-year model dependent
front brakes (V8 brakes are larger)
sway bars (V8 has larger front sway bar and 4 cyl doesn't have a rear swaybar at all)
8.8 rear end (4 cyl has a weaker 7.5 open diff)
V8 drive shaft
power steering cooler and all lines
V8 K-member (though the 4 cyl K will work, the pickup points for the V8 cars made the front track a little wider)
vacuum reservoir (4 cyl doesn't have one)
Vacuum tree (4 cyl is a little different-but will work)
battery and ground cables
V8 fan & shroud (4 cyl totally different and insufficient to cool a V8)
a/c clutch harness
clutch/flywheel/throwout bearing/fork/clutch cable
rack & pinion
...that's just a start. Pick out what you want to do with it and you can eliminate a few things. Honestly after doing several of 4-8 swaps, the last one I bought...which I still have...is still a 4 cylinder, and will remain that way. It's a lot of work to do it properly and finding fox body parts in this area might as well be trying to find unobtanium. There's not even a 4 cylinder car in ANY of the yards; haven't seen one in, gosh, gotta be close to 10 years--and I hit the yards up fairly often.
Mine is a turbo 2.3 from a Merkur and honestly it runs better than a stock 5.0 and gets +30 mpg, and is roughly 150 lbs lighter over the nose. If you can find one of those turbo engines out of a thunderbird, Merkur, Cougar, or a SVO Mustang, grab it...it bolts right in using the stock 4 cylinder transmission, drive shaft, rear end, etc. You will have to do some wiring changes, but it's real easy-took me less than an hour to complete mine and I work very slowly! stinger-performance.com has the wiring information and it's year model dependent and focuses on the LA2/LA3/LB3 computers as they are all wired the same.