The C-pillar and rear quarter area really is interesting.
It almost looks like the folks at the customizer's shop took a Gremlin/Hornet front window, turned it backwards, sliced it at a slight angle, then merged it back into the bodyside. The shape is a little different from the 1979 (onwards) AMC Spirit sedan.
I understand the car was powered by a Ford 351 V8 with a modified carburetor.
For the record, here's another image confirming Barris' involvement.