Small block vs big block questions

[Sniper X]

So, I have two trucks. one is the 78 Camper Special with a 454, all stock, and the other is a all stock 350 both are 4 speeds. The 350 truck is a 74. If I do do what I want to this winter as a project, which is remove the 454 and rebuild it, with rebuilt heads, bore and new bearings, mild street cam and headers. I already know It will never get more than 9mpg probably less.

If I pump up the 350, which would be kinda stupid now since it was rebuilt 7K miles ago by the PO....and do a 383 or something, what kind of mileage should I expect?

I am not "really after mileage"....more after good solid but streetable power and reliability. Just a little more than a stock 454 that is this old has.

I guess I am basically asking what you guys have for HP and mileage on some mild or wild motors, and how the reliability is on both the big and small blocks that are bilt up a bit or a lot...