If any one buys a replacement mechanical fuel pump from GSF or Veewee, then check the pivot pin ( this is the bit that goes through the pump and locates the arm inside) is fitted correctly, mine fell out today, van was still running but a bit poor falling apart after 50 miles use, the new pump looks better as the pin that goes through the pump is fitted tight, at least it got me to the MOT station.
This has been a problem with fuel pumps. Peen over the metal to hold the pin in better, use JB weld or replace the pin with something like a marine clevis pin with a split pin in the end, and a washer between the split pin and the pump body. Some people have used a bolt but ideally you need one with a smooth shoulder on the bolt, or the bolt thread will chew or be chewed by the moving part of the pump.