How much of a MPG improvement would I see, by converting a big block engine, to run on propane?

Ive been thinking of getting an older Ford Bronco, or Chevy Blazer (the ones with the mive big block engines), as a daily driver/offroader. My reason for this is that, ive seen some that are pretty cheap (less than $2000) that are fairly low km's and in good shape. The downside is the horrible MPG. Im wondering how much of a MPG improvement I might see on a properly tuned propane setup. Ive been told that propane is more efficent at higher compression ratios. Does that mean more power, or better mpg? Or both?