The point of rewiring the fuel pump is down to 20+ year old degraded wiring, even if you measured say 12v at idle at the pump once underway when the pump needs current(amps) this could drop seriously low some as low as 6 or 7 volts, result is not enough fuel and a possibly melted piston or two, also rewire the disaster of a fan circuit whilst at it, both easy to do, pump takes about 2 hrs, easy job cost around £10 or less in materials, same with fan rewire, the original set up melts the fuse for the fan and can occasionally cause fires!!
tabetha