I missed the mpg part - completely agree mpg increase on an existing engine will never happen. A turbo is a mild restriction in the exhaust when just freewheeling along and there's no way that improves mpg.

mpg can be improved along the lines Dave says + putting a milder cam in an existing engine (not something you need to do on a stock c body 360).

But a turbo can be a very efficient way to provide 25-50% more power than an engine has in it's NA state.

I ran stock Ford SVO/Turbocoupe turbos on my 383 when I first did it. They were too small, but they did work pretty well to get going. Something like that would work pretty good on your 360 (a pair of them). A pair of cummins turbos cfm wise is ok since you're a 5.9 at 2x rpm of a cummins, but the compressor map on those is pretty narrow and so it wouldn't be that efficient.