I can understand why you don't think it's the power valve. I'm sure the timing is correct since the owner seems to be turning over every stone?? But no matter what carb you install....if the big motor gets starved for gas (Lean condition) when you mash on the gas you're going to get a stumble. Go ahead and check all the other stuff that other members have suggested then if none of that works you can try the power valve. I had to try about 7 different power valves before I got rid of the stumble. You get pretty close if you run the car at idle in park with a vacuum gauge on the intake. The power valve "vacuum" number should be....watch this:
