4.3 V6 distributor won't fit your GMC.
The 2 HEI conversions I know of to fit the early GMC (as well as chevy 235) are:
Langdons Stovebolt, built from 2.8 V6 distributor. (small cap, remote coil)
Larrow & Sons, built from 250 / 292 distributors. ('75 remote coil, later coil in cap. Both big cap)
I've run Langdons for 2 years (about 6) am please with performance & look.
BTW, I,ve got a Mallory dual point,Promaster coil,cap, rotor & wires I'd love to part with!