If you're thinking about gas it'd probably worth getting a replacement carby rather than a top hat style of unit.
Carby and convertor $400 trade
Tank $400
Lines, cabling and Solenoid $200,
Gas Certificate $50
Labour 0 - $700 depending on where you get it done.
New cam profile $90
Won't be much change from 2 grand
If you're really keen you should score a 7K efi manifold and sidedraft the the carb on the front like an efi throttle body. You can even get sensors that allows you to control the system with an aftermarket ecu. I know someone who gets 7 L per 100km from a N/A L16 datto motor under hard daily driving with a EMS stinger. For the best effiency, it's best to get a new cam profile with different durations to suite aswell, not too sure what they change..
Check out http://www.gasresearch.com.au/, has a 20mb download about forced induction too, bit of a waste of time though for its size.