Intermittent Backfiring After New Head Gaskets

First off, thanks, folks. Y'know how oftentimes it's something simple...? After two weekends of frustration in 100 degree Texas heat...

Turns out, even though I had the plug wires clearly marked before tear-down, the passenger side plug wires were installed by my (still learning) 16-yr-old son. So either I marked the wires incorrectly when I disassembled it (always a possibility), or he mixed 6 & 8 up. She purrs like a kitten again!!!

As far as the head gaskets, I realize the 1008's are the 360 bore size. Pretty sure the correct 273 3.625" bore head gaskets are damn-near unobtainable, and have been for some time. Although I'd LOVE to have the 3.625" gaskets, I'm not having Cometic make me a set for a quiet cruiser. The previously-installed gaskets I found during tear-down were Fel-Pro 521 SD's, also for the larger bore, and it ran fine (except for the high temp). Did my research, and several members here have been using the larger bore gaskets on their 273's without issue, so that's what I was going by. Based on the cost difference, my hope was that I'd have better luck with the 1008's than the 521 SD or 8553 PT. My parts choices have been made solely for reliability and as much fuel economy as I can get, thus the tiny-runner SP-2P and 500cfm AVS2 (which, in my opinion, have turned out to be spectacular for the intended use).

AJ/FormS, the cam is stock 2bbl 273 solid lifter. Although the (true) 2" dual exhaust predates my ownership of 20 years so I can't speak to the mufflers, it's much quieter and tamer with the correct firing order. :)

Even though this thread is NOT about the high temp issue I've been chasing: Coolant recovery is a homemade sealed-cap bottle with the correct vented radiator cap. Radiator is a year-old Wizard with a (beautiful) hand-made shroud that I paid dearly for. Fan is 18" mechanical located halfway inside the edge of the shroud with a year-old Hays "short" fan clutch. Again, if it wasn't confirmed from multiple temp gauges I've tried that show the exact same temp, you'd never know she runs hot.

She's run like a Swiss watch since I installed Ray's recurved distributor and replaced the cheap plug wires with Taylors. I've driven/owned several 318's and a few 360's over the years, but this is my first 273, and I'm shocked by the bottom in torque this little thing makes with this intake and carb. Can't wait to put the valve cover back on and button up a few details to see what she'll do above 3500 rpm with brand-new valve springs. That's not the priority, but now I'm a little curious...

FABO for the save again! Thanks again for everyone's help on this. I've learned a TON from you guys over the years.