Have you tried the following, just fill in the on-line form for the part you want and await results and prices. I suspect your commodore is but a car/truck lump marinised so try these:
I've personally used breakeryard.com many times and they are brilliant although be warned, you can get a wide variety of prices and warranties and as you are dealing with second hand parts the maxim cheapest is best applies as most offer a warranty anyway, only differences might be you have to clean the part yourself.
You need anybody breaking Nuffield 4/60 465, 10/60 or 384 models.
The old tractor market is the only place where 3.8's are alive and well. You need to make sure you have 3.8 and not a 4.98 in the later Leyland tractors. Heads are different and you are right on the cusp.