I was told if the firing order is off, you won't get voltage to the coil.
I have had better luck with an AC Delco platium 41-993. Most of the 4.3lt have alongger spark plug hole than a S/b Chevy which also uses th R44 &R45 pluggs. If the truck is LP need to double check the vacuum lines for any leaks.
Check the distributor body for a plugged vent. This will let moisture accumulate in the rotor chamber and cause crossfiring. Latest plug spec for this engine is an AC R44LTS gapped at 0.035".
We are also having problems with the spark plugs glazing up.
Replace the plugs with 44's instead of the factory 42's.
Is the check engine light on?
We need to know the s/n so we know exactly which ign. / ecm the unit as
I just had a bizarre one a few weeks ago.
It was on a 3.0L GM in an H60.
Turned out to be the alternator!?
The only way I can figure it, is that it was leaking enough AC to cause the ign. to pick it up and missfire.
I only found it by accident. Was trying to track down some noise from the front (because of miss-fire I was working my way back to the timing chain).
Pulled the belt and oila, problem solved.
I honestly don't think this is your problem as NO ONE I know has ever seen this but I thought I'd pass it on anyway...
ahh the good old 4.3
ok first question !
what engine series is this ? is it pre vortec ?
check the engine pan, is it steel or aluminum ?
im asuming its propane fuel ?
if so what hours has it done since the valves were done ?
one of the most common problems with early 4.3 gm engines were the exhaust valve seats burning out, the heat the propane developes in the chamber far exceeds a gas / petrol fueled engine.
under load it gives the impression of backfiring but in almost all cases when this engine runs on propane and developes such a fault it was the exhaust valve seats
if you get the heads done be sure the exhaust seats are stelite material,
a quick compression check may or may not reveal this.
just one more thing be sure the ignition wire ends are coated with silicon grease inside the plug boots its a must for hei ignition systems