i went straight lpg on my a15 back in 2008, with a twin su carby set up. I dont advise it really and neither did revhead001 who is a lpg trained tech and Jmac, and lpg master of all things information based.

Tried it on my L series conversion with much faileure too (nothis changed on the set up except L16 not a15 engine.

A single set up with a larger throttle body adaptor (r31 throttle body with adaptor to the stock manaifold) to the manifold yeilded musch better results and driveability and power than the twin system ever did , and not issues with the lull in the paower range the su set up always had. Think it was issues with the signal draw and the pistons dropping slightly in the sus.

Good luck, i have my stwin su set up still as would like to nut it out one day but not really worth it when the single set up is easier to work with and drive.

You should fit a 66lt tank between the rear arches on a bracket too. no dramas there.