If fuel economy is the ONLY consideration, very difficult to justify a major change.
16 MPG highway with a six cylinder, medium compression truck, is NOT bad mileage.
The Carter YF MIGHT give you 10 percent.
The Weber, IF you change intakes along with the carb,
MIGHT give you 10 percent. If you use a 2-->1 adapter, you might LOSE
10 percent or more.
I do not know the percentage increase that higher compression would make (it will certainly make some), but doubt it would exceed 10 percent.
So do the math:
Lets say you drive this truck 5000 miles annually. At 16 MPG, you use 312 gallons.
Now increase your mileage by 10 percent to 17 and 1/2; and you still use 286 gallons.
So you didn't have to buy 26 gallons of fuel.
(1) how much to find the correct Carter YF, acquire a rebuilding kit (I can help with that
), and rebuild it?
(2) how much to find and acquire the Weber, rebuild it, and a manifold that will accept a two-barrel, manifold gaskets, air cleaner, etc.?
(3) how much to acquire new higher compression pistons, and rebuild the engine?
How many miles do you drive annually?
If the engine is needing to be rebuilt, then some planning when rebuilding the engine would be the time to make changes.