My '72 402cid as it sits gets 18.7 mpg. It is stock except for rockers, cam, headers, and intake. I had a '71 with a 350. Small blocks are great, but in a Monte with that HUGE engine bay they look tiny. I would also argue that the LS isnt the way of the future either. GM has already dropped them in favor of the new engines. The LS costs crazy money to buy aftermarket parts. Last i looked a decent intake was $900(its been a couple years). LSs also have no torque out of the hole, you have to wind them to make anything happen unless you add a turbo or supercharger(big $$$). If you want "take your foot off the brake and stomp on the gas till the tires scream" big block is the way to go. Even a mild, very driveable build will give you plenty of get up and go.