Thank you for your further advice. The advice I mentioned in my first post on this recommends a relay. As you point out, I can also put the HEI to the ignition switch. I will figure it out and do something, not soon though, so I have time to think. There will be room on the terminal post of the ignition switch when I have everything else off it if I want to go that way.
One instruction showing the need for making an HEI unit accommodate an electric overdrive is given by poster ray_mcavoy in 67-72chevytrucks.com. I find that in a search for "HEI overdrive" in that forum. I plan to follow his instruction.
Thanks!
1950 GMC 100 Panel Truck, 1958 270, 3-spd Saginaw with BW OD, 3.73 12 Bolt Differential from a 1966 C10, Front Disc Brakes Bought it in pieces 9-1976, driving since 6-1978 Follow in the DITY | 
