Need some help, rear end swap

[Devil Pup]

Alright so I have a 1969 C20 with a 350 and th350 running 4:10's in the rear. I plan on bagging the truck within the next few months but i want to change to 1/2 ton and 4 link the rear. I want to increase my mpg and take strain off the motor on the freeway. I dont have a tach but when running around 60-65 on the freeway it sounds like the truck wants to blow up. "literally" I wanna know what 1/2 ton axle "out of what year model truck" would you guys reccommend and what gear ratio i should look for. "I wanna swap a 700r4 for the trans but that depends if i have the cash to do it at the same time. but I will be doing a 700r swap for sure in the future. just a matter of time. anyway the plan is to bag the truck on 20's possibly 22's in the rear, I wanna know what the best rear end/gear ratio would be to put under this for cruising and mpg. Thanks for any help guys.