Will a 94-98 MC fit a 89 fox brake booster??

The master cylinder will fit. You may need to adjust the booster push rod a little to get it to work correctly.

See 87-93 Mustang 5.0 Brake upgrade pages. Improve your 60-0 times! for the best and cheapest brake upgrades and 5 lug swaps. The site doesn't sell anything but some fittings, but the tech notes are great. It tells you what junkyard or remanufactured parts to use to upgrade your brakes. It has almost everything you could want to know about Mustang brakes on it.