If NG is a "better alternative" then it should be able to stand on it's own merits in the marketplace.
Why would you suggest government interference in the form of taxes, instead of capitalist market forces? Why would anybody want to use taxes to control any aspect of society?
Taxes are the problem, not the solution.
Funny you should mention taxes because my thought was to actually REDUCE taxes on gasoline sold at stations that provide an NG pump. Say .05 off the gasoline tax to facilitate building the infrastructure.
This would achieve two goals ... first, a modest tax reduction on gasoline, and second, provide a market advantage (be it short lived) to those gas stations that take the risk and invest in the NG equipment.
If you reduce taxes the right way, it can be part of the solution.