EFI and gears do contribute a lot to mileage, an LT1 swap with the 4 speed OD trans would pick u pa lot of mileage over what you currently have.
BUT the LS engines are way above and beyond the gen 1 stuff- especially in stock form.
Take the 5.3L, swap it in and you'll instantly pick up power, mileage and driveability. Then down the road swap the cam, throw on some headers, swap to an LS6 intake and port the stock heads and install LS1 intake valves- you know have 450+ flywheel hp and STILL have better mileage than when you started.
There is nothing better about the old stuff than the new stuff.