Hi, That all sounds reasonable. If you have a C6. Maybe a Gearvendors Over/under drive could help. It's a basic 2 speed transmission that replaces the tail shaft on the automatic. You end up with twice as many forward gears. Should make it above 20 MPG easily with your other improvements. The only down side is a shorter drive shaft is needed to make it work.
Dave 69 Torino Talladega, 79 Ranchero Street rod