If your breather hose from the drivers side rocker cover is connected to the inlet duct, remove it ASAP. It should remain connected to the port on the side of the throttle body at all times. The way the LR breather system works this is the best way. Fitters that hook a breather into the front of the mixer think that the LR breather is the same as Holdens and Fords with a PCV system, which it isn't. As Philip said, you need the tee piece with the small vent to the inlet manifold. The size of that breather sets your base idle speed as much as any other component. Go get a genuine one, they're not that dear (I paid $18 for the last one).
The problem with moving the breather is that the engines dirty fumes are being put into the throttle body in a way that they enter the IAC valve and goop it up. With the stock breather only on full throttle do fumes enter the throttle body and they are drawn through on the front side away from the IAC so don't enter it. At low throttle the fumes are drawn into the engine via the small branch of the tee. It's really quite clever and LPG fitters do love ruining it.
This is a home made brass tee, I had to reduce the small hose outlet with an internal jet of around 3mm to correct it.
http://www.aulro.com/afvb/attachment...1&d=1341968784
Note the mixer, it's one of mine at 46mm ID, see below.
As to the mixer ID, the maximum size that a 3.9 can run successfully is around 46mm. Most of the off-the-shelf mixers are around 42-44mm and will reduce your top end power a bit, you may lose say 500 rpm off your power band. Down low you'll never notice it. I've re-profiled plenty of mixers in the lathe to increase their ID and make sure the gas outlet channel is as clear as possible.
Oh and do get some o-rings for the MAF, they're easily lost so keep a spare handy. I make mine by super gluing cut down larger ones out of a random collection I once got at a garage sale. You can probably buy bulk o-ring rubber at a bearing shop.
