But that's not to say they won't technically work.
I believe some of the late 3.8 mustang blocks and 4.2L f-150 blocks are actually superior to the supercoupe block in strength. Some models came from the factory with main stud girdles.
You can drop a 3.8L SC crank/rods/pistons in any mustang 3.8l or f-150 4.2 block however, regardless of year.
The factory non-supercoupe 3.8L/4.2L pistons and rods aren't quite as strong as the SC stuff in theory.. But I built a '94 SC with nearly 40k miles on a 1996 mustang 3.8L block/crank/rods/pistons that would indicate otherwise so far (knock on wood).
Of course, if you have an 89-93 SC you will have to stick with an SC block, as none of the f-150 or mustang blocks have the provisions for the early style crank sensor mount. 94-95 SC uses eDIS ignition and the crank sensor is mounted to the front cover.
Jeramie