Finally some Dany's Mag swap progress, and fuel line routing questions?

Have you tried duplicating this on the street?
Does your fuel tank have a vent?
I am still running stock 5/16” fuel line in stock six cylinder routing in my 75 Dart Sport. Not same body as yours, but supplies plenty of fuel to run mid 12 second passes with a 400 cube big block. I doubt your fuel line itself, size or routing, is the problem.
I had the same symptoms on two different GM F-body cars. Both were immediately, although temporarily, fixed by removing the fuel tank filler cap. Long term I had to add a vent.