Hi Rich,
I couldn't get the three pole relay to work on my '73. I tried as John76 suggested with switching the wiring, but it didn't work and I just swapped back to the 4-pole relay which matches my original wiring. I think you also have a 4-pole relay on your car, so that's what you need, not the 3-pole one. The square one that I found on ebay was oddly enough original equipment on Polaris Snowmobiles back in the day it seems. Weird. Got it from some guy up in Minnesota (Blunt land). You can find them on ebay. But $$'s 😒
https://www.ebay.com/itm/165858301843