Yes if listed for '99 Explorer it should fit '99 Mountaineer.
This is just one opinion but pre-bent lines are a pain to install, more work than bending your own as you string it, because you have to remove so much off the vehicle to put them on and may still end up bending and unbending to get there, and that bending is harder to do with stainless. I'd only consider them for a frame-off restoration.
I have not used them on an Explorer, but I've done a few runs with CuNi aka NiCopp tubing. It's really easy to bend by hand, just make sure you use some form (if not a made for purpose, tubing bending tool) if trying to do a tight radius bend. By form I just mean something round like a floor jack handle, piece of pipe, baseball bat, etc., and try to avoid the cheapest generic tubing that has wall thickness less than 0.028", which is also harder to flare without buckling in on itself.
You don't try to recreate the entire shape of the run before installation, just sort of string it and bend as you go since it's easy to bend. When it takes an *interesting* path (like ABS module to the right front wheel), I find it easier to estimate the length with just a little to spare, by taking some 12ga solid core household wire and roughly stringing it along the same path to get a length approximation, then cut the line a foot longer than that (including an extra ~2ft for 3x approx 2.5" diameter coils near the ABS for strain relief instead of flex pipe), or if you're meticulous about doing it, can cut the line just a few inches longer then have already flared the other end. Do put a cap or masking tape over the end so it doesn't pick up rust/debris as it's strung along.
