Question on fitting modern trans in 71 Demon

The 518 is not the best way to go. The best way to go in today's Mopar world is the GM 200 4r transmission where there is no cutting of the floor and using a wilcap adaptor for the conversion seems to be what others are doing. The ratios are 2.74, 1.57, 1.00, 0.67 so using even a 3:23 rear will get you out of the chute a little faster than the 518 and the 200 4r is stronger.