The neat thing about the Mopar OD trannys is they kept the 6 position switch into the tranny, instead of adding another 7th position like GM and Ford. I know classic Chevy owners need special shifters when converting to a 700r4 because the tranny input has that extra postion.
The OD in the 500/518 is electronically controlled, and when activated, the 0.63 OD takes the place of the old 1.0 third gear.
I'd love to hear all about your installation, 1974Cuda360, and any hangups and whatnot, because as soon as I get my A518 and "test dummy" 360, its going in!
When I do this to my Challenger, I can document EVERYTHING, so that everyone can see how its done.