A 9inch from a 65-66 or a 67-70 Mustang will fit with a slight modification to the spring perch. The widths are slightly different between the two sets of years, I believe the 65-66 are closer to the Pinto stock width.
I believe if I remember correctly that a Maverick rear will also fit with little work, but again very rare.
However, these aren't that cheap
Might be cheaper to go with a Currie or Moser rear that is made to your exact measurements.
Or get one out of a pickup or other model Ford and have it narrowed and spring perches installed.
I don't think there are any 8.8 rears close to fitting without mods, and none came with leaf springs. I personally hate these rears anyway, so I don't pay attention to them.
Lots of people will tell you that an 8inch would be plenty. I am of the opinion that more is better and overkill is perfect.
When you say C4 manual, are you talking about a manual shift valve body in the trans, as the C4 is an automatic.? My end goal is now a Toploader with a Gear Vendors overdrive.
Russ