3.8/3.9/4.2 swap wouldn't be that hard, just the computer, harness, matching
tranny, and 1983-86 Mustang 3.8L motor and
tranny mounts should just about have you there. (There's a little more, like exhaust, fuel line, and brake line issues, but just get the pieces from a 5.0 and swap where necessary.)
2.8/2.9/3.0/4.0/4.0sohc swap is a little more complicated, you'd have to track down mounts from a 1979-1980 2.8L Mustang... that's the only major complication, but it's a considerable one, good luck finding the mounts.
To be brutally honest though, the 5.0 V8 swap is EXACTLY as difficult as the 3.8/3.9/4.2 swap, and you'd probably be MUCH happier with the end result (more reliable, more powerful, comparable fuel economy). The cost difference would probably be negligible as well, perhaps even cheaper, given the number of 3.8/3.9/4.2L engines vs the number of 5.0s built, and the survivabilty rates of each engine family.
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1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4
tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".)