You'll be really lucky to get 17 to 18 mpg with the 350 and vortec heads with the 4.10 gears. I've ran that setup before my LS1 and it ate the gas. Without the 4.10's and a good tune, bored-descreened MAF, Helix spacer, headers and exhaust got me 20-21 MPG. When I installed my 4.10's and cam it went to 14 MPG.