I'm working on my 63 Dart GT that's getting a 5.7 Hemi. It's getting to be transmission time and was wondering what is the best way to go? I have heard guy's using 4L65E's, NAG's, 727's, how about the 545RFE's ??? They bolt up to Hemi's, actually have 6 speeds (2nd Prime), what's the down side?