If you didnt run a box i would have said module in distributor .
Had a similar issue on customer car last week , ended up it was the ignition coil . Was a blaster SS coil . Car would fire and run fine cold then as warmed up it would miss and backfire , even pop out exhaust . Initially thought box, but after checking everything tried a spare blaster II . All good

Tex


New best ET 10.259@129.65 .
New best MPH 130.32
Finally fitted a solid cam,
stepped it up a bit more
3690lbs through the mufflers
New World block 3780lbs 10.278@130.80 . Wowser 10.253@130.24 footbraking from 1500rpm
Power by Tex's Automotive