I've installed two Heidts units. One in a pro-street 454 BB 41 Chevy in the mid 90's and the other in an early hemi powered '53 Chevy PU in 04. Both are still on the road with no problems. The '41 used stock Mustang 4 cyl springs and the hemi used the V8 springs. Both used stock Mustang upper and lower "A" arms, spindles, and strut rods.
What you have to look out for is the Chinese repro crap. They are making repro "A" arms that are junk and they will bend as Deuce said. Also any Mustang IFS will be much more reliable if you do NOT eliminate the strut rods. If you spring for one of the units with tubular "A" arms without strut rods make sure you box in the lower section of the crossmember where the lower "A" arm bolts on. This area has been known to crack if its not boxed and the strut rods are eliminated. Not sure if Heidts has had that particular problem but its a good idea... just in case.
If you are going to install a Mustang II IFS then Heidts is the best IMHO. There are others but its been my experience that Heidts fits very well and has the correct geometry.
"When buying a used car, punch the buttons on the radio. If all the stations are rock and roll, there's a good chance the transmission is shot." - Larry Lujack