Car takes ALOT to move it...

I've got the same problem with my 340 727 Duster.

Have you gotten the car up fast enough to shift into 2nd and 3rd? Mine won't go into 3rd. I adjusted the kick-down linkage and it helped some, but I'm still left with problems. My converter is 2500. It's my first non-stock one. But from what I've read, a "high stall" converter shouldn't change the drive characteristics of a car/tranny. It just allows you to hold on the brake and rev higher when you come off the line. Not sure on that, but that's what I've read. I'm no tranny guru.

Although a new rebuild, my tranny was sitting around for 2 years before I got it in, so my problem may be age-related.

Don't neglect hooking up the linkage. Shifting manually is no answer either. Hook it up.