There is actually a notch in the crossmember for 4-speed cars. You can just make it out in this pic of my factory 383 4-speed Dart.

7671833-MVC-001F.JPG (114 downloads)

The funny thing about science is that if you change one miniscule parameter you change the entire outcome to the way you want it.

JB Rhinehart, Realist

A-Body's RULE!