Suddenly I found my long lost "Rewiring Round Tuit". Somehow it has been AWOL for way too long....
Anyway, the OEM -77 wiring harness is now in a cardboard box, together with the alternator, reg/ rect, etc.
So now I have mounted the M-Unit, a new starter relay, and the Vape reg/ rect. And just made and connected the main wires between battery and M-Unit. I do hope all voltage drop gremlins now will be history, and there will be more room in the headlight bucket.
I am sure there will be a lot of headscratching and re-re-wiring before all is done, but now there is no going back (I think)