My bone stock 97 silverado with vortec 350 averages 15.5 city and highway combined. It will get almost 18 on the highway if conditions are right and I don't drive fast. My 06 silverado with a heads/cam 370 and 4.56 gears gets 15.5 down the highway and it's not a mild build by any means. A mild 6.0 with blow your 350 out of the water for both fuel economy and performance. No brainer in my opinion.