He's probably got the rotor phasing way off when he installed.
The teeth are 60 degrees distributor degrees apart on a GM HEI, and if you don't clock the gear correctly, the rotor nose will be anywhere up to 30 degrees out of alignment with the spark plug wire terminals.

Without a power relay and proper size wiring, the HEI is starved for current.

You also have to contend with the fact that a good portion of the guys that do the distributor change to an GM HEI get the distributor in 180 degrees out anyway...
One thing about a I-6, it runs nearly as well with the distributor in 180 degrees out as it does in correctly, just some power loss and some backfiring...

If it 'Backfired' through the air cleaner, it's probably a fuel problem.
If it 'Backfired' through the exhaust pipes, it almost always ignition.

So many cats, so few recipes...