You might get some help on the Stovebolt.com forum. They have Engine Shop and HiPo Shop subforums and there are discussions about HEI, Petronix, etc.
Tom at http://www.langdonsstovebolt.com/store/#!/Ignition-Systems/c/18665949/offset=0&sort=normal sells one. The 235 Chevy distributor does interchange with the GMC. An early Toyota Land Cruiser 6 cylinder distributor also fits.
Toms kit works perfectly on a GMC. I made a mount out of bent steel for his coil to attach to two screws on the side plate. If you buy his kit remember to put 2 very short cap screws into the side of the head where the coil was. It will leak oil from those holes and too long of a cap screw will hit a push rod. A large cap HEI will work but are a very tight fit and I use a 1/8” flat side plate to do it on another engine.