66 gmc 250 carb upgrade recomendations / gearing

[66gmc250]

my truck currently has a Rochester 1 barrel and i had rebuilt it but it still leaks due to a previous owner striping and jb welding were the fuel line goes in to it and was looking at getting another 1 barrel but ppl have recommended me to get a newer style but i don't want anything that would rob my mileage i want something to improve it


and i also know i need to change my gears in the rear b/c i am at about 2500 rpm at 55 and think i need to change it down but i don't know what to get it changed to