Know this probably is not the reason for your 400 problem but it was for my 400 and was fixed rather quickly after a thorough methodical search of the disturbed area. I had a guy do some drive line work to straighten up and properly set pinion and drive angles and replace my driveshaft. I took the vehicle out for a test run and it would not shift, I did not rev it past the point of ruining something. Limped back to the garage and explained what happened and after a lengthy search it was found that the vacume line running down to the front passenger side was not secure. Secured that line and everything was like it was before, just a simple vacume source, again hope this helps. Thanks HUGH
