You can convert the GMC distributor to HEI very easily. I had mine done by machining the points cam lobes down to fit a Ford reluctor wheel on it, then use a Dodge pickup sensor bolted down to the points plate. Mount a 4 wire GM module on the firewall or intake, use small HEI coil mounted same. Got a spare 270 distributor handy? good luck. Jim. A good write up in Hamilton Fuel Injection on Binder planet..
http://www.binderplanet.com/forums/showthread.php?p=799563#post799563