Please help me before I lose my sanity
 

So the basics.... 1974 dodge d100.... 440... 2x 600 edelbrock carbs.
Truck was running 7.7 flat in 1/8th before off frame resto. While she was apart we put in a new cam... same that was in it. 509 purple shaft. Had block vatted and crank checked out. Rebuilt carbs, new timing chain, changed to aluminum water pump and swapped out painted headers for ceramic coated headers. And added msd 6al box and new tack.
Ever since we've gotten her back together this is what happens
We can go out and get in her and take her up the road... she cruises all day long fine.
If we start pounding on it... she will make 3 or 4 good pulls then after that.... it starts
She will launch hard... shift to 2nd... still pulling hard... then it momentarily (for maybe 2 seconds) will just fall on its face. But then picks right back up and has full strength again.
We've changed the fuel pump (mechanical) 8 times.... added an in line pump... we've ran a line inside and at full pull she still has 3 to 4 pounds of fuel pressure.
Had alternator rebuilt, changed back to chrysler ignition.... same thing...
Changed plugs... same thing
Changed back to msd.... changed distributor. .. put on bigger fuel lines... o.m.g I've lost count of what all we have tried and still it does the same thing....
3 or 4 hard pulls and she does perfect .... after that it falls on its face ...
Any ideas? I'm tired of throwing parts at this thing....
Thanks in advance

I would think it needs a minimum of 7 psi fuel pressure. Have you got good ventilation on the fuel tank?

3/8 fuel inlet on the carb's and from the fuel tank.... :)

Welcome.

Everything points to fuel line/pickup restriction before pumps.

C -

In addition to what the guys were saying, don't rule of the possibility of being over carbonated. 1200 cfm is a lot for the 509 stick. Some one will chime in and tell you that the old 6-packs have more flow, put the street version have a vacuum progressive linkage.

Assuming you've done what was suggested above, if you have a spare single 4V manifold, give it a try. A quicker fix might be adding FOUR BORE 1" spacers under each carb.

Archer