I've got a '63 Galaxie with what they say is an original 390 in it - I'm wondering - What's the average HP and MPG I can expect out of something that size?

As an alternative (to rebuilding the 390), I have a 302 (1975) that's ready to go.

For comparison purposes, what's the HP/MPG I could expect from the 302?

In addition.. I know the 90 is a big-block, while the 02 is short - what sort of conversions (etc) would I need to link everything up..?

Don't bother with the swap. The '75 engine is an inefficient low compression 135 hp gas hog. Your mileage won't improve much, it will just be a lot slower. Also it would not bolt up to the existing trans, and the engine mounts would be a bit tricky.

That swap would obliterate the value of your Galaxie, too.

Your car should get 15-18 mpg on the highway. The engine has 250 gross hp, worth about 200 hp in the modern 'net' rating.
If your mileage sucks, pay attention to a proper tune up. Set up the points, timing and dwell properly. Use solid copper core plug wires (intended for an industrial engine) no the modern carbon core wires.
Better yet, get an electronic ignition conversion kit.

Also, the 4bbl carbs get better mileage than the 2bbl, if you drive modestly.