Hi TwoUpTourer,
Swapped the converter over today - all went well as expected - everything is very tight and it's a pain but all done. I ended up putting a VE commodore converter in which is a little different to the VZ one but fits fine and I think is a bit better the way the temp sender fits in as the sender doesn't enter the coolant but gets heated through the aluminium.
Pulled the old converter apart and found multiple coolant leaks into the screws that hold the converter together, plus a perished and holed rubber membrane which was undoubtedly the original problem There was no sign of corrosion so a seal kit may have been fine. The new converter was only a few more dollars ($275 versus a seal kit for $200) so I'm still happy. Checked the screws that held the NEW converter before installing it and found three that were loose. I recon the rubber membranes compress over time and the screws need to be nipped up. The OEM LPG booklet that comes with the car says this in fact but it doesn't appear in the service schedule so it doesn't get done plus its a real pain to get to.
Cost to take the heads off and put new chains and guides in...$1500. $500 to get two heads surfaced and crack tested, $600 for a complete chain kit with guides, sprockets and new oil pump .... bought it from the USA on ebay...same kit as a Cadillac SRX 3.6 but a fraction of the AUS price, $300 for a set of Cadillac SRX gaskets ... complete top end and front cover gasket goop and seal kit including valve stem seals. The rest was new head bolts and other bits and pieces. Also cost me 3 days of my life and a lot of swearing.
Replacement engine was $1500, so $3K all up because of a misdiagnosed LPG converter problem - could have been worse but I just hope even one other person can be saved from going through this path by reading this.
So, I now have a completely sound second LPG engine with new chains gathering dust. The replacement engine was very low KM but is a not a factory LPG engine, not that it makes much difference according to most.
Live and learn.
Regards