Keith, I just remembered something... A long time ago, I had a problem that was very similar. I would drive the car a good 20 or 30 miles, then it would start doing lean surges, and once in a while just quit. After messing with everything more times than I could count, I got pissed one day and drilled a hole (yes, I can hear you guys laughing now, lol) in the gas cap. It never happened again. It ended up that the venting gas cap went bad and wasn't venting, and after the motor would run for quite a while, it would start vapor locking.
That sounds like what could be happening to you, and could help explain why your A/F starts to change. I know you said you have a vent in the tank, but perhaps you could look into making a change where you're 100% sure the tank is venting.
BTW, that was with a Holley Red pump on a 350.
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