Gerry A.
I'm with Auto10x, I am in doubt about the power valve idea. My understanding of what a power valve is - is that it controls the flow of fuel to the secondaries when they are not in demand, under no load condition the engine will produce its full vacuum potential and will pull the power valve closed to not allow fuel to flow through the jets for the secondary circuit. If you have a power valve that requires 10" of vacuum to hold it closed but your engine can only create 8" on a good day then yes you will run rich (I'm not a Holley man so correct me if I have this bass-ackwards)however, my thought is that before you would see the backfiring in the exhaust you would flood out or foul plugs or blow excessive amounts of black out of the exhaust or have gas in your
oil pan (Do you?). Also I believe it was stated somewhere here that this backfiring is apparent at any load level (full throttle or idle or cruising) correct me if I am wrong. If this is the case I would think that this would eliminate the power valve as the engine would "clean" itself out when you open the secondaries for a few or more moments.
Another suggestion would be to have your engine builder use a degree wheel on the cam PRIOR TO taking the thing apart and verify that the lobes were ground correctly according to the specifications from the manufacturer of the cam. Even the manufacturer recomends that you do this with the installation of a new cam.
Are you certain that you don't have to much lift for the piston/head comb you have? You could have too much lift which would cause your valves to hit the pistons in turn bending the valve head and not allowing the to seat. Again though this does not make sense if you do have a consistant 175 psi in all cylinders.
I post my opinions based on the experiences that I or friends have experienced first hand. I may be way off base here as I know there are a lot more experienced people here than me, so take this for what you can.
In closing I still have not heard anything that would convince me that it is not valve train related.
Don't let anyone push you around when you pay good money for something.
Have a great weekend and I hope that you figure out what is going on so you can start enjoying your ride.
Again, keep us posted when you finally figure it out, this is the way we all learn.
Thank you.
John