I dont believe a mechanical pump will supply enough fuel for your engine @ WOT. I ran an old Holley electric pump on my 512 other than the noise I never had any fuel issues. Never even ran a regulator and/or return line.
What type of fuel pressure gauge do you have and where is it located?
What size fuel lines?
It is possible, With the right pump. On the engine masters show Freiburger just made over 1000 hp on a BBC with a tunnel ram and two 4s naturally aspirated, all fed by a mechanical pump.
