I did this with mine. Was an original 6/3sp manual. I converted to 5.0/AOD. There is a whole lot to change. At least: rearend, motor stands, exhaust manifolds, radiator, front sump oil pan, transmission-besides the engine. Most of this is Mustang unique. Then you get may as well go discs in front to match the 5-lug rear. May as well put an Explorer rear because it's cheaper than a 9". Dual master cyl. Exhaust vs brake lines. Fuel line. Steering changes. Oil pump while pan is off, timing chain while cover is off...... I should have left mine alone. The 6 was fresh rebuilt when he hit a deer.
As far as donor cars, if you want carbureted 302 maybe find an LTD or Grand Marquis with low miles and use that. Maybe add a small cam and intake. Or later 80s vans if you can find a conversion van with low miles. It may have a roller cam. Get a cam eccentric and mod the front cover for fuel pump and front dipstick. Swap intakes.
If you want fuel injection then there are a bunch of cars out there with 5.0 EFI engines. Mustang, Cougar, Lincoln, Marquis, T-bird, F150, Vans.... All have pluses and minuses. Most have lots of miles. Might find a low mile little-old-lady car or conversion van that is a rustbucket with a good engine though with some patience. Lots of articles on this. Computer behind the dash, new fuel tank with a pump.