not an expert but...
I'd check the O2's blms to see what they are doing.. maybe the o2's are bad. Using a laptop with some autotap program or comparitable.
Try using a fuel pressure regulator to see if it can fix the problem?

A walbro 255lph is a good size pump.. I dont know what its rated for but im sure it'd handle 600 horses no problem. It'd probably exceed the capability of your fuel lines.
All that walbro has to do is maintain X pressure.. 38 or 42 or whatever. I dont see why that pump is particularly dying in 3rd gear... it doesnt matter what gear its in or what speeds, rpms etc.. all that pump has to do is maintain that pressure.... and im guessing u dont nave huge injectors which makes the pump work more keep the fuel up

I'd definately get it on the computer to be scanned...