Drive train not quite right

The stall of a 904 converter that came in most of those cars was a lot higher than a 727 converter out of a heavy van. There were a few heavy Dodge vehicles that had hi-stall converters, but they also had low geared rearends. This situation you have is a common occurrence when switching to a 727; if the trans is from a heavier vehicle; low end suffers from decreased stall; giving the 727 a bad rep for "parasitic loss"...