Wierd Question

As to your little misfire Gremlin .. no idea. Could be almost anything. Welcome to Ford quality.

The 3.8L is a solid engine. Not the smoothest, quietest piece out there, but it holds up pretty well. It will easily handle more than the stock horspower. There are folks running around 240-260hp naturally aspirated with cam swaps and the like, and forced induction folks with superchargers pushing well over 300-400 (although they generally have beefed-up internals for over 300hp).

As to the compatibility - while it looks like the same engine on paper, there are actually some very significant differences between the Windstar 3.8L and the 3.8L in the Mustang.

The big issue is that the Windstar is FWD, the Mustang is rear. The Windstar and Mustang blocks are not the same, and are not interchangeable (different bellhousing pattern on the FWD blocks to accommodate a FWD tranny, etc.). The internals (rods, pistons, crank) would be generally interchangeable. I suspect the heads would be too with some minor adaptation. The lower intakes are interchangeable with some minor modifications as well, but the upper intakes are very different and are not. The rest of the componentry (from accessories like alternator, AC, etc., to hoses, brackets, tubing and wiring) are probably not very interchangeable due to the fact that one engine is FWD and has to fit transversely into a minivan engine bay, while the other sits longitudinally in a Mustang bay.

If you're talking about performance upgrades for a Windstar, something like a Mustang UDP would fit, although you'd probably have to drop the engine to be able to install it. Cam swaps are a biggie for the 3.8 now, and are yielding +30hp easily. Any popular 3.8 Mustang cam would work in a Windstar, but again, you'd have to drop the engine out to get to the cam. Obviously things like CAIs and exhausts designed for the Mustang 3.8 wouldn't fit because of the FWD packaging.