Pastorpat wrote:

My son and I are looking to place a 1996 mark viii IRS in a 1971 mustang.  Wondering if anyone has seen it done.

Not in a Mustang.

I'm putting one in a '64 Galaxie (actually Country Sedan wagon) and I've known folks who have put them in F-100s as you've noted.

The MN12 'Bird/FN10 Mark rear suspension is pretty wide.  If I remember correctly your stock axle is 60.25 over the flanges.  My wagon's rearend was an inch wider than that, and I still had to have my rear wheels made with an extra inch of offset to accommodate the rear.    You might get away with it.  I think - have not tried it - the subframe could be narrowed maybe 2in in the center, where the diff mounts.   

The MN12 subframe is quite bulky and heavy (80lb or so for the bare subframe - the early SC subframes have more reinforcement inside, Marks and other 'Birds did not get this.)   Mine is not a bolt-in; I put pie-shaped cuts in the front legs of the subframe to narrow them 2in, lower them 2in, and move them back 1/2in which more or less centers the mount bushings on the frame rail just behind the point at which the rails kick up to go over the axle - I'm fabricating weld-on mounts for the frame rails at that point.  The rears barely fit between the frame rails; I'm fabricating a new welded-in crossmember to pick those up.  I'll also have to deal with the gas tank, which won't fit in the stock location due to the subframe and sway-bar location.

The '98-03 Cobra IRS might be a closer fit, it should be just about exactly the right width, the tubular subframe is less bulky, and the mounts would be easier to adapt.  They're more expensive on the used market but I think they're all 3.55s and Trac-Lok whereas only the SC 'Birds got 3.27 Trac-Lok gears and all the Marks were crappy 3.07 open diffs.

Pics a little later, have to run right now.

Last edited by JEM (3/14/2012 12:57 pm)

Pastorpat wrote:

I did some measuring and the with of the front spring perches is 40 inches.  Each perch is 3 inches wide.  The subframe rail mounts on the inside of the front spring perches, then it gets wider until it reaches the rear of the carn.  The front of the Mark VIII mounts are 41 inches wide so i dont think that will be to hard to make weld up brackets for.   The Gas tank does seem to be the issue.   I will need to cut down the mark viii cage.  More details to come and some photos.

My biggest concern there is that while the front subframe legs are the proper distance apart, they slope downward to get there; in order to readily mount the stock pads under the Mustang spring hangers you won't be able to get the axle and suspension tucked up enough under the car.

I may be wrong, but I'm guessing that to get the whole thing far enough up under the car you'll have to cut off at least the front mounting legs and fabricate new ones out of 2.5in x .125 steel tubing to extend down and under the frame rails.

Just a guess.