Thanks Leroy. I looked at those and read the forum post suggesting they were superior to the Wilwood. While they appear to be a higher quality I'm not convinced it's a matter of quality but more one of size/capacity.
Wilwood don't state what compound their pads are comprised of but they appear ultra hard and mine glazed very easily. I thought I would make some up of softer compound and increase the area while I was at it. I was hoping a better CoF might help.
I cut up some Corolla pads and fly-cut the thickness down - Gee, didn't that blunt the tip very quickly!
I machined a spigot on a new locating plate and welded the locating plates onto the pad backing plates.
The extra pad thickness required an extra 12mm spacer.
The performance is much improved but still not capable of holding the car against 1st gear, however, as I now understand it, that requirement is in lieu of 1:5 test slope being available.
The reason I'm going to these lengths to keep using the Wilwood is that a lot of engineering has gone into the cables and mounting brackets. If, like you, I was still in the design phase and knowing what I know now I would go with the later, larger, Wilwood mechanical brake. Better still I would see what drum in hat options I had.