First off, the mods you mention probably won't improve your fuel economy at all.
As for which engine to get, smaller is generally better.
If you average the "real world" MPG of v6 vs. v8 Chevy pickup owners, I bet you'll find that the average v6 MPG is better than the average v8 MPG:
https://www.fueleconomy.gov/mpg/MPG....make=Chevrolet
And before you dismiss "a few MPG" difference as inconsequential, realize that the difference between 16 and 18 mpg* over 20,000 miles a year is 139 gallons. At 3 bucks a pop, that's $417 a year. Every year.
*(I don't know if the numbers will show you actual averages of 16 and 18 mpg - that's just an example based on a quick look at the lists at that link. You can crunch the numbers yourself.)