Hi Jonesie,
I believe that in order to run an engine efficiently on pressurized bottle gas, the carburetor venturi is required to be a larger size than when using standard liquid fuel. Also, obviously some type of pressure regulator should be utilized in the delivery set up. Also the length of the intake runner can get to be a problem that is caused by the refrigeration action of the gas. Perhaps a bit of 'fiddling' with your current fuel delivery system would yield better performance rather than attempting to re-invent the wheel and a whole new set of problems to sort out.
BC1
Jim