[Pyrotechnic]

Have you tuned your carburetor to adjust the air/fuel ratios at idle/cruise/power/WOT? Did you use a wide band oxygen sensor and a vacuum gauge to do it? Can you tell me the air/fuel ratio's under all those conditions?

Can you describe to me the exact timing curve, static, mechanical and vacuum in your distributor? Not what the manufacturer paper says, but what has actually been verified in real life with a timing light or distributor machine.

Is your cruise RPM at highway speed optimal for your engine?

Those guys with the 700 cubic inch big blocks getting great gas mileage you describe will definitely answer YES to all those questions. Or at least the engine builder or tuner will be able to.

If the answer is NO, you can swap in whatever engine you want but it won't help, you will get less than optimal fuel mileage and performance.

I don't know if your engine has been modified at all or fine tuned, but fuel economy is the product of a well thought out engine/drivetrain combination that has been fine tuned to run efficiently as it possibly can. You can apply these principals to your engine to make sure it is giving all that it possibly can.

The easy way out is the ever popular LS swap. The engine/transmission is already very efficient with zero modification. As long as your rear gear ratio and tire size is within range to work, no problem. Pay a tuner to program in the last extra bits of power/efficiency with a few mouse clicks and it's done.