G'day Tricky,
yes there were two Holden conversion kits sold commercially that I have come across, but given the amount of conversions done I am sure there would have been plenty of others about as well. Then there would also have been plenty that were either made up at home, or made by the local mechanic etc..
The two I have are the Johnson kit which has a cast iron adaptor plate and then the Dellow kit which has a cast aluminium adaptor plate and utilises the heavier Land Rover flywheel, by redrilling it to bolt up to the Holden engine with an adaptor/spacer, and also uses the Land Rover starter motor.
The conversion engine mounts for a Land Rover that had a four cylinder engine are fabricated from 8mm plate, while the ones for a Land Rover six conversion were cast iron, but I have seen fabricated examples of them too.
If your engine mounts look much like these here on the right, and the adaptor plate is cast iron, it's possible you may have a Johnson kit.
Diana, Lotzalandies on here, would have a lot more knowledge on what commercial conversions were available back in the day.
Cheers, Mick.

