20 mpg out of a chevy high horse power motor is possible... I have a 72 C20 custom camper pick up, with the stock 402 4bbl carburetor and 3.54 gears in the back and I got 17 mpg towing a tandem axel trailer with a 71 Chevelle on it. It is possible, but yes the later small blocks with tpi or tbi injection systems should work well with your needs.