confusled

dodgy feed from ignition feed via 40 odd year old plug??

take a spare spark plug with you...if/when it happens again quickly pull off lead from any plug and rest spare plug on engine,connect lead and see if you got a spark..

have a 12v bulb test lamp with you too to see if you have power at coil etc when happens if you discover you have no spark..and carry wire (thinish) with two croc clips on to supply feed from battery to coil etc...ie carry stuff to fault find when it happens..
and i suggest that just cos you got a fuel loop tank back to tank dont mean the fuel cant vaporise...should not..but..

odd really because the only thing I was keepin after engine swap was ign arghh lol.

its a new live ignition feed btw