Something isn't right..

So I have a customer with a 72 Dart I talked him through building a Dana 60 and he has 456 gears. He had a 904 rebuilt with a new torque converter.. not sure of the stall but it's not super significant but not stock... He had the bottom end done at a machine shop 30 over, Pistons maybe .010 below deck.. we put a 8553 felpro head gasket and some $400 eBay Edelbrock open chamber heads.. a pretty nice Edelbrock air gap intake and some cheap Summit headers. A 7al with a brand new MSD no vacuum advance distributor with the light silver and blue spring..
We broke the engine in but it was very lean and lean popping anytime we took it over 2500 RPMs.. I was obviously a old boat motor carburetor. It was a Holley double pumper. He wanted a new carburetor anyways so he bought a brand new Holley brawler... It ran super lean and this is with a brand new stock fuel pump..
Okay starts perfect idles fine (ruff with a long duration cam but fine..) turns right off and revs very responsibly in park..
Put it in gear and runs fine again a little rough but that's mostly due to the cam..
Oh yeah hydraulic lifters and stock rockers we do have the stuff to upgrade but not yet.. we've also tried changing power valves as it has about 12 to 13 lb of vacuum and the factory power valve was a 4.5 and we put a 6.5 in... Plenty of adjusting got it a lot richer and it helped quite a bit but the big problem is once he gets going or starts giving it serious gas it chugs and pops and chugs and pops...
It almost felt like it was running out of gas.. I went home and grabbed an electric fuel pump that I have the same one that I use for my dual quads and it brought the fuel pressure up and steady..
Same problem..
Thank you for any help..