A 5.0 Fox makes a perfectly acceptable DD car, especially the hatchbacks (for when you need to carry stuff). I DD my '84 and, aside from the fact that my particular car is a bit too slow and the exhaust a bit too quiet for my liking, I'm happy with it. Gas mileage is pretty good, too (24 MPG highway average).
The issue I would have against your idea is DD'ing a NICE Fox. Seems like such a waste to me.
Given, neither of my Foxes are show cars, but I prefer to keep teh Notch for my weekend toy and DD the '84 because 1. Four-eyed auto hatches are a dime a dozen out here, 2. The CFI motor is crap for building up, 3. I have enough parts already invested in the notch that if I DD'ed it and some butthole without insurance destroyed it, I would be totally screwed out of all that (whereas I have almost nothing in mods on the '84).
When you have a nice car and a nicer car, the lesser of the two should be the DD, IMO. If you're filthy-rich and can afford two really nice cars, that's fine and dandy, DD whatever you want. But it just seems a shame to take a really nice Fox and just use it for a daily beater. A Fox doesn't have to cost $3,000+ to look nice or be a good DD - usually those in the $4,000+ range are more than just solid, they're usually either built or they're well-preserved, in which case DD'ing it would put it through normal wear and tear that will undo all the effort put into preserving or restoring that car. I know there's millions of Foxes still on the road, but there's not that many really NICE Foxes out there, so tearing one up by DD'ing it (no matter how well you take care of it) just seems ... I dunno ... kinda tragic.