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ya it could be somewhat lean on the low end. or its a bit rich, and the plug is half fouled out. the fact when it idles it seemingly idles smooth would lead me to believe if you have an airleak, its a small one. usually any airleaks causes a really erratic idle, and when ran is followed by the words, Man it never ran the that before. it was running like a raped ape and then just quit.

the carb could be faulty, or the gas cap vent may not be working properly too.
Those plugs most of the time are an optional plug and should work fine.
CMR7H is what most are using. you may want to try a brand new plug 1st to see if it just isn't giving out for some reason. If the new plug makes no difference it would not hurt to check your carb settings no matter what.
Check your settings by turning the screws in untill it just bottoms out - Do not over tighten - and counting the total number of turns. Maybe something got tweaked or turned on accident.

They should be close to these numbers:
H = 1 1/2 (+/-) 1/4 Turn out.
L = 1 1/4 (+/-) 1/4 Turn out.
The farther out the richer the farther in the leaner.
Good luck.:)
Hey gunner.....you said you had a kill switch on your truck too...is there a possibility that its not working properly....causin you engine to run funny??
read somewhere else that someone had a problem with theirs...i dunno.im just thinkin out loud
stripped the intake and sealed the gaskets, i think the problem may be the bolts holding the airfilter on may be a little long and when done up tight and the engine gets hot its push the insulator causing a leak. I have added some extra washers and have yet to to it out. but hopefully this wil resolve the problem.
thats a very likly cause. good thinking on the heat expanding the iso block.
i take it you have the stock plastic block?

they can warp, which your may have.

a cnc manifold is a good thing, but again you have to watch the bolt length.
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