Usually cars of this age have been modified / changed / bodged at some point . If you need to replace the feed to the sender unit then you can take it from any non permanent live.hope this helps in some way.
just a word of warning sender units nowadays usually send a negative signal to the guage ( an earth wire should be connected to one side of the sender) be VERY CAREFUL and check that you get it right as sender units are made of wire wound resistors and if wired wrong could get HOT.