Small block to big block?

Go big. You may or may not need a new radiator as both big and small blocks have the option of WP inlet on either side. BUT, the radiator may not be up to cooling your new engine. We have a factory 340 car and have built a couple of 440 cars. I don't build anything with huge cams, etc. The difference between the 340 and 440 is tremendous. There's just really no comparison on the street even if there isn't a huge difference in quarter mile times. As for fuel mileage, not much difference there, either.