Hopefully your interior isn't...
Hopefully your interior isn't as bad as this, but still, when budgeting your next Fox move, keep in mind some interior restoration is likely.
The milder climate of the West means the cars are driven and driven and driven. Our '91 hatch, for example, has a few bolt-ons, 186,000 miles on the odometer, and has never had the valve covers off. It's still in daily--albeit easy--duty. It also has to pass an emissions sniff test every two years, so it's scheduled replacement powertrain features mild upgrades. If a power-adder is to be fitted, it will likely be an E.O.'d centrifugal.
One "power-adder" we normally don't recommend is an engine swap. Years ago you'd see the odd 460-powered Fox; they proved to be way too much. A 351 Windsor is a vastly better choice; in a stroked version (392 to 427 inches), they out-power-to-weight a 460 any day, plus out-handle the nose-heavy big-block beast, even at the dragstrip. But the 351W swap requires more small parts than you'd think, and all things considered, a blown 302-347 makes more sense.
Speaking of strokers, opting for a 310/331/347 short-block when renewing a tired 302 works great. We say the stroker engines feature more internal friction and likely won't go 150,000 miles like the stocker, but for hobby use, such concerns are secondary.
Transmissions are a non-issue for the bolt-on builder, but for an up-powered Fox, you'll need to renew the gearbox. For the ubiquitous T5 manual, the options are to rebuild it for around $300 or replace it for approximately $1,300. Naturally, the silky little T5 is over-torqued by today's hot-rodded engines, which is where the Tremec TKO 500 or 600 five-speeds come in. These transmissions are named by their approximate horsepower ratings, so they're much happier behind a supercharged 347. Of course, they'll cost you an extra grand, around $2,100 or so. This doesn't seem to stop many of Brother's Performance Warehouse customers: The company estimates 75 percent of its buyers opt for a new Tremec rather than a new or rebuilt T5.
As for the AOD automatic, they're definitely a job for a specialist rebuilder. By the time you're done rebuilding and modifying an AOD, however, you'll likely find moving up to an AODE/4R70W is the better choice because the later transmission offers all AOD features, but it's stronger and more tunable.
We think of rear axle gears as bolt-ons because so many are sold, but if yours are stock 3.08s, a set of 3.73 gears for street cars and 4.10s or higher for strip cars are a must. In fact, these are the best bang-for-the-buck modification as nothing else will get you moving as fast, and especially not for so little money. Installation is a pro job, but you can save a couple of dollars by having the gears changed when the rear suspension arms are fitted and the rear axle is partially disassembled anyway.
Ultimately, modifying a Fox today is done because you either have one already or are looking for a fairly hardcore performance toy. Both are great reasons, and the parts and expertise are widely available to build whatever Fox you desire.
Topping them all, Fox parts are relatively inexpensive and there is no better bang for the buck. Our caution for street-driven Foxes is to budget for the interior and limited exterior restoration these cars typically need. Dedicated track toys are another matter, and in that case starting with a fairly dog-bit car makes sense as long as the chassis is straight, rust-free, and all of the mechanicals are there and salvageable. To paraphrase the old T-shirt, "Old Foxes never die, they just go faster."
Which Fox Is Best?
Fox Mustangs were built from 1979 to 1993, but the electronically fuel-injected '86 to '93 are by far the best for hot-rodding. Typically the '86 cars are not considered good performance candidates because they used a restrictive cylinder head, but as an amped-up 5.0 will use aftermarket heads anyway, this isn't a real issue. Mass air cars are the best of all: '88-'93 California and '89-'93 50-state cars.