OD tranny hard to come by?

An A500 or an A518 is pretty much the same from the main case back (i.e. The OD section). With either you need to do floor surgery.

FWIW, I have an A833OD in my Barracuda. With 3.55 gears and 27" tall tires I am turning just over 2000 rpm at 65 mph and getting 22mpg with a 370HP 360.