[1969400HO]

A backfire is a symptom a vlv that is opened when ignition is applied to the cylinder.
I would at this point remove all 8 plugs.
Put the (I Think is) 15/16's socket on the crank bolt, manual trans in Neutral if have that, and rotate the engine one way then the other a little bit maybe 1/8th turn back and forth,
looking at the Dist and see if there is a lag - slop . like a worn out Timing chain exhibits.
1. Drifting timing
2. Backfire thru open valves.
3. Symptoms of a fuel delivery somewhat.

I am very suspicious there on those parts to verify Timing chain & Gears mounted properly.
New from a previous buil, long ago doesn't mean "good" or proper!!

And considering it was mounted like new, Yet NOW has worked its way loose with such few miles possibly? ?
This is a easy Quick fairly simple thing to do.

Usually electronic will NOT exhibit drifting, they usually fail 100%.
Or maybe try another Distributor all together also..
Again, If you had a BORO-Scope could probe into the TC area and look at things.
One last thing, What does the gear on the camshaft look like?? Is it intact of possibly being Ate up for some reason.? ?

A failing TC setup would definitely affect the fuel delivery centric roller and cause weak fuel delivery so poor operation at higher RPM's. yada yada
So if you verified fuel pressure then that Might point to what I focus on here some.