I also would love infor on upgrades others have done or had luck with on their Big Cars. I am working on getting a factory style exhaust for my two 71's, asl well as wanting better sway bars. Hard to find stuff that is specific to these years.
My 71 Fastback is pretty tame, started with a very sad, tired, hammered car. Saw lotsa off-road use. Original base 302/3spd/AC/gage and console equipped car with no PS or PB. Pain to drive and stripped interior. Car was cheap and (mostly) running, going to the $700 car-crusher program 13 years ago. Got it for $300.
Friend gave me a 4spd Toploader and 9" from a 68-69 Cougar, this diff is a little narrower than a 71 so helped eliminate some tire rubbing. Another friend hooked me up with his neighbor, whose kid had just stuffed the family Hot Rod into a ditch, a 71 Coupe that was pretty nice before it got bent. Netted me a nice 351W, PS and PDB, really nice doors (mine were hosed) as well as most of my interior. Later found a 72 Cougar with a ruined front crossmember/ suspension that yielded working power window system and a tilt column, as well as a drivetrain which was sold for more that the car cost.
Where I'm going with all of this is look for the donor cars that may have bits you can use to trick out or improve your car. Buy'em cheap, get what you need and ship the rest. I was thinking of EFI next (86 T-Birds with V8's can be had for cheap), but bought a 71 Vert that needs exhaust, so that comes first.
Also, don't overlook pickups, sometimes lotsa good bolt-on motor parts in those.
Good luck, keep your eyes open, and glad there are more than a few of us with the Big Mustangs
The car:
Best regards,
Tom