as far as I know the cat was only introduced when ULP was due to it killing us if burnt without one.
I doubt straight LPG systems need a cat, check out the deicated LPG Falcons and have a look underneath...should shed some light
Usually there is a slightly different cam profile for a dedicated LPG engine, but Im sure they will be able to make one custom for you if required. Should give Mock a call and see what they have to offer. Apart from that you will have to overcome the sump issues of the AU engine...would probably be easier to fit the 4.0i EL engine as I think they use a similar/same sumps as the EA. the car will have to be setup on a dyno to get the proper advance curve etc but im sure you will be able to see a fair few horses out of straight LPG.
May I ask why you want just LPG? so the emissions become redundant?
Lets go find that Hotdog tree I planted
Lowered EF Futura
Black Steelies ( too cheap to buy proper mags )