There is a gas mileage thread floating around here somewhere, but a fair estimate would be 5 miles per gallon out of your gasser. It might be more, it might be less, but I wouldn't count on getting near the double digits if I were you.
CNG could be a viable fuel, but right now it isn't. The infrastructure just isn't there yet. Many municipalities DO use CNG, but they have their own filling stations. Range is limited by tank size on CNG vehicles and the problem is only compounded by the lack of filling stations around.
A more viable solution is to run on propane. Propane has many benefits including higher octane, cleaner burning, and lower cost. However, it does have some downsides. Even propane can be difficult to track down at times. It also contains fewer BTU's per gallon than gasoline meaning it will take more propane to go a given distance than gas under the same conditions.
Of course there are ways around that problem, especially considering you own an SB1 powered bus. In order to balance out the lower thermal energy in propane you MUST build the engine around its octane advantage. This means running much higher compression and a more aggressive timing advance. A small block Chevy purpose built with say 12.5:1 compression, a pretty wild advance curve, some decent flowing manifolds (centerdumps) or headers, and good heads such as Vortecs (ported and polished since fuel atomization isn't a concern with propane) topped off with a good intake should make lots of power and still deliver good mileage.
Building a purpose built engine has some limitations though. You will not be able to run dual fuel unless you replace all the rubber seals and lines with Viton or other alcohol safe materials and run E-85 (similar octane and energy properties to propane) as your secondary fuel. Even doing this there will be still be sacrifices as I've never heard of a dual fuel setup getting the same mileage as a purely propane setup will.
As far as the conversion...well...aside from the motor stuff it's easy. You will need a tank, high pressure LPG lines, a lock off, a regulator, and a vaporizer like an Impco 425, all of which is commonly available on eBay or through various vendors.