Highway RPMS ?

my brother got the best mileage out of his 76 Feather Duster (had 3 spd trans not OD) after the carb went to shyt and I had piece one together from 2 different 1945s/ the original and one from a '74. that was ear;y 80s, so I dont remember what was bad about the original, nor what pieces came from which carb. I just remember going to a junkyard and finding the only 1945 they had (small hole in the wall yard not many people knew about) and hoping I could do "something" with it. I was "just barely" driver license age at the time.