1971 c20

[tubbed55]

Hello, I have a 1971 C20 they I want to make into a daily driver. This truck is a 402 big block, auto trans, A/C Power brakes, steering, cab lights, the works. When looking for a truck I wanted a c20 with a small block but fell into this. Now I am at a cross road.... Do I just rebuild the 402 or take it out and put in a small block. Only concern would be the gas milege. I do not drive a long ways to work but do travel 4 hours to see my parents and 3 hours to the deer lease several times year. Is the big block gonna make that much of a difference in the milege..I know they will both suck....But if Im talking a mile or two differnece I would rather have the big block torque. I also thought about either a gear vendor overdrive or a 700r4 to help. What kind of milege should I expect to see bewtween the big block and small block. Sorry for the long post but need to make a decision.
Thanks, Ronnie