11.5 volts to pump shouldnt make it splutter but could be better[thats providing your alternator and battery are good]
run a good quality thick power cable to the pump[use an msd kit if you dont want to make one up-its not expensive] also make sure earths etc and any relays switching your pump on are fresh
it also helps to have a known good condition,quality fuel pump too

these are very old cars now and any replacement of the wiring done correctly always helps-some of the wiring was marginal when new-after all the years it doesnt get any better