http://www.diyautotune.com/
http://nistune.com/
g`day mad max, i`m mad082`s tuner mate, these are two websites that you should look into, the megasquirt does do a stand alone
ignition ecu that can be run of a vn commodore map sensor to do your load and is cheap. the niztune on the other hand is a little dearer but has the abilaty to use all your stock sensors and you can just zero out all of the fuel settings in it, i myself have done this
with a straight lpg rb25det and it work fantasic.
hope this helps, gary, Gympie Mechanical & Dyno.