Propane actually has 107% of the energy of gasoline on a mass-basis (ie, BTU/lb), which is how AFR is calculated. Since it is a less dense liquid than gasoline (68%), a gallon of propane has about 73% of the energy of a gallon of gasoline.

You will never get the same mpg fuel efficiency with propane with a carbureted propane system but it's not hard to get 75-80% of your car's gasoline economy on propane. However, the cost of operation should be much lower if you buy low cost propane. In Ontario, the cheapest places to buy propane is where the taxis fill up, which is around Pearson Airport.

Since HD-5 propane has an octane number of 104, you can safely run a higher compression ratio (eg 10.5:1) without any risk of knocking. Higher compression ratios increase the fuel efficiency of ANY internal combustion engine. See Fuels Forum: Compression Guide.

Mixer-based dual fuel engines will make less power than a mono-fuel engine because of the added flow restriction of the gasoline carburetor. However, both may be tuned to provide the optimum fuel mixture.

Someone out in oregon had converted his slant six truck to propane and used the MegaJolt/E ignition system. He found that his truck ran better on propane than gasoline and needed a lot less timing than commonly thought.
See Slant Six Forum: My timing map....