knarf:

Given that the change from D-jet to K-jet occured in late 1975 (for the 1976 model year USA delivery) it seems that there is a possibility that the 1975 heads which you received were made for a K-jet engine.

While you had the engine apart did you take any pictures of the heads?
One way to tell the difference is by the pedestals on which the rocker arms pivot. If the heads were made for D-jet they will have manually adjustable pedestals for valve clearance; if for K-jet the pedestals will be hydraulic with no means of adjustment. This assumes that the heads which you installed were obtained complete with cams, bearing stands, rockers, pedestals, valves, springs, etc.