start / no start gremlins 67-72

[RustyDan]

Anybody have or had this one...? INTERMITTENT SPARK...?!@#$%
72 chev pickup. Mostly original. Was a Utah truck 72-05, California now.

(My Dad was original owner, He got A.L.S. And I promised him I'd care for his truck after he passed. Now I love the ole rusted beauty. Son was to help restore, but he's a Marine now).
So here's the skinny...
New (original) engine, trans. with usual goodies. Original wiring harness. Usually starts perfect, then occasionally NO SPARK, Turns over fine, But no go. Have HEI (my install) with dielectic grease under chip. Stock type everything else.

Tried many things, Wire traces, Ground checks, Harness plugs, Ign. switch. Still comes and goes.
I heard it could be lower voltage feed vs. higher voltage feed to distributor. What???
Haven't confirmed that one yet but would like to.
...here comes the humility...
I found that a "light" tap with a deadblow to the uncovered coil usually makes fire again.
(no pride in that move). Not dumb, just feel like it.
I'm learning too much patience here. Any advice welcome, No matter how obvious or complex. I dig the info.
Thanks man....
Onward... thru the fog.