update
BVCBR's post put me in contact with him, Jamie Wagner, at MSD.
I gotta say JAMIE'S TECH SUPPORT IS THE BEST I'VE EVER EXPERIENCED.
Multiple emails have explained why the MSD distributor is outputting a signal which will drive an analog tach just fine, but is almost unusable for Racepak rpm input. I am down the path of installing a MSD Digital 6AL Ignition Controller which has a tach output with the square wave the Racepak needs. But also Jamie has provided me with tach signal convertor 8132 which should work on the GM HEI system i am replacing (this is in case i go back to that system)
Some TFA household complications are slowing my controller install & testing, but for the 1st time in 2 years i feel i understand the problem and likely have an answer.
to be continued...Karl