Ford conundrum???...

I've been helping a customer do a frame of restoration on a 72 Ford highboy. He pretty much did a copy of what I was doing with my power wagon. At any rate he had a 360 rebuilt by my engine builder. Complete stock factory 30 over rebuild.. we put headers on it the factory intake with the cheap Chinese two barrel and did a Chevy one wire distributor up front...
The break in went like clockwork and everything was running absolutely perfect. The timing was 12 initial and 32 all in,.. he was enjoying the truck and everything was going well. Running starting driving everything.. he starts it up the other day and it was idling kind of low and anytime he tried to give it gas it would stall... I instantly thought it was a vacuum leak. I tried to talk him through all the different scenarios and possible problems over the phone and of course give him a hard time about the cheap Chinese carburetor....
So he called me out today and we started going over everything. I was really feeling a fuel delivery problem but everything kept checking out. I could get it to run at idle a very slow seemingly rough idle and anytime I gave it gas just tried to slowly rev it up it pops better shoot out the carburetor install... I pull the carburetor open it up and see nothing going on... I checked the fuel pressure from the fuel pump and it's fine. We pull the valve covers and crack it over to see if there's like a stuck valve or something and nothing everything was fine... I take the distributor cap off and look underneath to see if I have a broken rotor or something remind you everything is brand new...
Nothing. And as a matter of fact the first thing I did when I walked up to the truck was put my hand in a distributor and try and turn it to see if the timing got off and it was Rock solid...
Completely exhausting everything I knew as the last ditch I should grab me a sharpie and I marked the distributor to make sure if I turned it that I could put it right back where it was as I had personally timed this and knew it was dead on. I advance it a half inch and it runs perfect????
HOW IN THE HECK DOES THAT MAKE ANY SENSE?..