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.