I have a '56 Crown Vic with 292.  It used to have an automatic but now has a Mustang 5 speed with 5th gear being a 1.25:1 overdrive.  My rear ratio is probably 3.30:1 and I would like more like 3.90 or 4.10.  Any suggestions where to start hunting?  Prefer to not purchase just a ring and pinion gear since I don't know anyone who will assemble it being this old but if I can find one, I can work from there.

I found some posts here on the subject but all pretty old..