Dbrods has a mechanical pump.
I believe your 42lbs will struggle to make 500hp. The 255lph will probably have a ceiling of 500hp before you see a drop in fuel pressure. I know with a boost a pump and 60s a build struggled to keep idc under 110% above 500hp. The 255 will help the 42s stretch it a bit, but I think 450 will be the safe limit.