Note, re: older thread in Ignition forum: I originally had a Holley Sniper on the engine, but found that there were just too many variables/failure points to chase with it, so I replaced it and the EFI fuel delivery system (inline pump, Edelbrock sump, pre and post filters, return line) with an Edelbrock 1406 carb (fuel tank->mechanical pump->filter->carb).
Since then, I have also replaced the oil catch can with a PCV-to-carb line and a breather cap.
I added a Ford-style starter solenoid, along with a 4-pin HEI relay.  For now, I'm bypassing the NSS.  A push-button actuates the starter, and a keyed ignition cylinder sends current to HEI+ via the relay.  I put a 30A fuse close to the battery on the HEI feed.  See HEI wiring diagram.  I've checked voltage with a multimeter before and during crank.  Full voltage before crank, and slight dip (~11.0v) during crank.
I'm using overhead console ignition controls instead of the column switch.  Because of this, there is no auto ACC power kill during crank.  The gauges remain on.  I can bypass them and see if that slight difference makes a difference.  Seems like the motor should be able to fire at slightly below max battery capacity, though, or else I'll be jumping it on a regular basis.  
The distributor seats to the block, bare metal to metal.  The block is already slightly oxidized there because of the humid weather.  I can brush off the oxidation and see if that makes a difference, but I hope not because it's just going to oxidize again.
I'm going to add break-in oil through the spark plug bores today, readjust the valves, and reset timing to 10-degrees BTDC, being sure to remove any possible timing chain slack before another crank.  It's a new chain, though, nice and tight, and a new harmonic balancer.  I don't see how anything could have slipped.
Thanks for all the ideas so far.