1966 burban

[61memo]

Hello everyone im new at this site and I need help with my project.My suburban is all original 283 v8 and manual 3 speed.Im doing the disc brake conversion kit but im keeping it 6 six lug.The problem I have is I cant do more than 55 mph on the freeway i was thinking of changing gear ratio but the cost of doing that is over 600 dollars im on a budget,is there a cheaper way I was thinking of swapping out the rear end with something newer I would like to keep the 3 speed manual and keep low on rpms on the freeway thanks