Greg,
It is a little hard to say. I have been working under the idea that the Vincent single leading shoe need a big boost in the coefficient of friction to wake it up and make it work. However, with a big 4 leading shoe you probably do not need as big a boost in the coefficient of friction, because all of the shoes are working as hard as they can. Thus, the Porterfield RD-4 that I am testing might be too grippy for a big leading shoe brake even though its grip at 600 degrees F is better than cold. I have also talked to Porterfield about Australia and they do not have a presence there.
Ideally, if you could find out the coefficient of friction of the existing linings Phil is using and that would allow you to make a better choice because you could pick a lining with better fade performance and stay near to the original design specs for the coefficient of friction.
If you cannot get that information, I think the Ferotec D3920 would be the logical place to start. Here are some of the details:
When looking at the numbers, I believe when they say "hot" they are referring to working temperature which is around 212 F. To see what happens at blistering hot go to 400 F. You can see it does drop. And, look at the line in the middle graph that shows the second fade... it is still pretty good until above 550 F. The shape of the curve is pretty good, showing that the coefficient gets better until it gets stinking hot, but it always stays pretty high.
This is pretty much a guess based on the premise that AM4 was a lining that was used in the big four leading shoe brakes during their day. The closer you stick to the AM4 coefficients, the more likely it is that you won't have grabbing problems.