It's just like anything else that you'd attempt to do to a car.
What are you willing and capable of doing?
I've used the upholstery. I used the mach-1style in the last project. Changed the front sport seats over to SVO foam, and covered the rear seat to match.
Firstly, they are not as easy as "slip on". The upholstery fits very tight and has to be rolled on ( Think puttin' panty-hose on your seat).
Nextly, there are several anchor bars that slip into pockets sewn into the seat upholstery that have to be hog-ringed to the seat frame as you do that. There are two particularly tough vertical bars that "blind anchor" through holes in the frame that hold the seat back tightly against the frame.
There is an entire thread on here somewhere where I recovered the front seats in that SVO seat style in technicolor w/ really close-up shots that make my arms look like they are so hairy you'd think I'm a primate.
I managed, but it was far from easy. But when it was done, they looked great.
The only complaint I could have if I had one, was that the seats "sat" higher than I liked. Even the SVO foams made me feel like I was an inch too high from where I wanted em to be.
**Here,..I even dug that up for you......for free.
http://www.stangnet.com/mustang-forums/threads/svo-seat-upgrade-on-fox-sports-seat.859208/
And people think I'm not helpful around here......sheesh.