sorry to hear this
when tt would hit a wall, how did it feel, like it progressively just ran out of steam?
im having the same kinda issue on a EFI motor

it just hit a wall at 4000 rpm..with the all in timing at 24 degrees.....
Almost ashamed of myself now,,,,,I used a Harbor Freight light with the advance dial set on zero, instead of my trusty 40 year old craftsman.... I have a timing tape on the balancer. I had the kid hold it at 3k and set it at 36 degrees. I guess the dial on the light could have moved, as it turned very easy, had to squeeze in there, with the power steering pulley 2 inches from my head. I'm 6'" 2 , 250,,,,going to pull it shortly, will advise ,,,,,,

I'd pull the valve covers and have a look before I pulled the motor.

Are you sure timing is fully advanced at 3000rpm ? Not many distributors are, no matter the manufacturer.

Even if manufacturer or friend tells you this you still need to find out by yourself. Many merc modules go to 4k before fully advancing. I know of one that is even more than this. 525SC,

Here's one thing for sure..
400hp takes about 32gph of fuel flow.
Either your flow meter is off, or you have a fuel restriction somewhere.

Other things that concern me. You mention having a timing tape. Has anybody ever verified the TDC mark on the tape with the actual piston TDC? Degreeing the cam and verifying TDC timing pointer are 2 things you HAVE to do on a new build, otherwise much tail-chasing happens when an issue occurs.

Did you ever verify the rpm where full advance comes in? Should run her up to 4500 no load while watching the timing marks to make sure there isn't a high rpm jump in timing. Take note of the rpm where it stops advancing, too, but also double check for unexpected movement up at your max rpm. Bench checking the mechanical advance and knowing the full range is also important. Ignition boxes can add their own advance. Know your system.

Fuel pressure AT WOT should be identical to fuel pressure at midrange or cruise on a NA carb setup. Verify that first, then also make sure there are no flow restrictions in the carb.

You bumped your timing and the motor liked it, but if you were already experiencing fuel starvation you really doubled down on the lean condition.

M