456 gears

Look at the fuel system first. Stock fuel tank or a cell. If stock, what's the sender line size?

Put a mechanical pressure gauge on it that you can see all through the run.

I wouldn't run anything less than a -8 or 3/8 line from the back, better yet a -10 or 1/2 line all the way up. 250gph pump minimum.

Something has changed drastically from the dyno session. Headers, carb... something. What's different from the dyno session?