After 2 solid days on this problem I think I've got it figured out. Figured I'd add to the thread below to hopefully help somebody in the future.
http://67-72chevytrucks.com/vboard/s...d.php?t=621876
My Retrosound Hermosa has been fine the past couple years. During my recent 9 month rebuild project I changed a bunch of stuff around including a new, although cheap, HEI distributor from Rockauto so I wasn't expecting it to be the problem. After getting the Blazer back on the road and working on the bugs I noticed the audio would cut out, but only while the engine was running. On researching I found a bunch of infuriated people, and it seems HEI and MSD ignition related RFI is not an uncommon problem for these units.
http://www.retromanufacturing.com/pa...cle-is-running
In my case, it turns out the little 2" HEI coil ground wire, under the dust cover, was too small. It was tight and looked good but it was 14 maybe 16 gauge, I didn't measure it. I had an old distributor that I took the coil out of and on comparing the two found the old coil had a much larger ground wire, so I slammed the old coil in the new distributor and the problem seems to be fixed.