Something isn't right..

If the engine is still assembled, I would check the CrankCase for vacuum/pressure. And I would defeat the Vcan. Then I would start pulling in timing, irrespective of the numbers. But I would NOT go WOT until the Power-Timing is verified.
And, for testing purposes, I would back the rockers off to zero preload plus just enough to shut them up, max 1/2 turn. No adjustable rockers? That's a bad sign on a build of this caliber right away.

You mentioned big cam several times but offered no specs. How big is it? and does the car have headers?

I would be doing a compression test pronto, and since I have the tool, probably do a LeakDown test as well. One or both might lead to checking the Cam's split overlap, which I would do anyway.

I would not try to diagnose until I knew the basics. I charge more because I'm worth it. If the guy doesn't like it, move along; there's a cheaper guy in every garage. Keep notes. Blackball the losers and creeps.
I'd rather be known as the guy who fixed it, but it cost a lot;
than the guy who worked on it for hours and hours, charged cheap, but the customer had to go back several times, and in the end, he never did get to the bottom of it. Thatchit gets around in a worse light than the former. If you fix it, the customer soon forgets how much it cost; but he never forgets that you fixed it.