The '67 to '70 'Stang housing is 2 inches wider than the Mav
and the spring perches are only 1/4" wider, so a little nudge
to seat the springs is all that's necessary. (I've done it)
Remember the extra inch on each side when choosing wheels/tires.
You have several other options to choose from:
1. The '65/'66 'Stang housing is a bolt-in (same width) but
good luck finding one!
2. Lincoln Versailles is also a bolt-in and they have disc
brakes. Not too tough to find.
3. If you're going drag racing, and are even considering
slicks or sticky street tires, a wise investment would be
a Currie H.D. or X.H.D housing. They're good for 700+ hp,
and are literally a bolt-in. (currieenterprises.com)
The factory 9" housings, especially the '67/'70 'Stang housings
aren't known for their strength in severe performance usage.
I've already twisted an axle tube right out of the center with
only a 302/c4. Also, the perches can shear off or wrap around
the axle tube. (yep, did that too!)
On a factory hsg, at least reinforce the perches. 2x2" steel
plates welded from each end of the perch to the tube does the
trick. 1/4" steel is plenty strong.
I got tired of breaking housings and went with the Currie
unit 3 years ago and haven't had an ounce of trouble since.
For about $300, it's cheaper (and stronger) to do it once and
forget about it. Once it's painted no one knows the diference
unless ya tell 'em!