You're in luck, the 2.8 has a one-piece rear main seal (mustang II 2.3s and 302s used 2-piece seals).
Clymer's Mustang II repair manual suggests that you install two small sheet metal screws into the rear main seal, and use a pair of pliers on each to pull it straight out. Installing the new one is as simple as lubing the inside with assembly lube, and the outside with fresh motor oil and driving it in using a drift of the appropriate size (a socket or piece of pipe of equal diameter will also work well enough in a pinch).
The only bad news is, that regardless of what kind of transmission, you've got to pull it out, as well as either the flywheel/clutch or flexplate/torque converter.
While you're at it, I'd replace the torque converter seal/input shaft seal and the output shaft seal of the transmission while you've got them out, and if it's a manual
tranny I'd replace the clutch and resurface the flywheel while I was at it, no sense in putting it all together now just to redo it later because of another part failure.
If you weren't all the way up in Wisconsin I'd be very interested in buying it, IIs down here are generally well above market because dirt-track use has nearly dried-up usable cars, as well as the fact everyone from up north comes down here to buy our generally rust-free/low-rust stuff!
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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".)
1978 Mustang II V-6 
