other idea
blank out the mech pump and fit a eletric pump with a regulator if needed
you would need to go down this route if you go for bike carbs any way
you got me thinking so i read back trough some of my uni books and older car pumps i.e. pre 75 era used a spring to keep the arm one the cam and a linkage that had lost movement (elongated hole) so the diaphram could remain in positon under pressure until the cam came back around.
later on a return line was used and fuel pumps changed
3-5 psi should be about right
somthing like one of thease
http://www.rallydesign.co.uk/index.php?cPath=281_282
and one of thease
http://www.rallydesign.co.uk/index.php?cPath=281_283