hey my friend has a 71 challenger with the stock shifter with a rmvb and all he did was grind the D to 2 detent down so the shifter would slip in with out pushing the button on the 1 to 2 shift then the next shift will be 2-3 the button must be still pushed like going from 2-1 on stock trans .
This seams to work great and smooth he has had like that along time with many differant motors in the car and the car now runs in th 9's in the 1/4 with no cage stock appearing interior and all steel body except for fiberglass hood .