Well, it's on there but I'll probably be doing it again because there is actually some flex in the 3mm thick steel plate. I will have to drill new IS mount holes and use the appropriate IS-PM 203 Shimano adapter, instead of this concoction of homemade IS+43 plate and IS-PM 160 adapter.
I also found (even without braking force applied) that the 3mm plate brings the caliper slightly too close to the spokes and there isn't enough outward adjustment in the PM slots to centre the caliper over the rotor, so I did a lot of grinding and filing to get it "perfectly" lined up.
Do NOT use Stupid Shimano XT precision engineered bloody racing brakes haha, there's no clearance for doing the build by hand. Be sensible and use Hope C2 #3 if you're going to mess around with a homemade adapter plate.
(Hope C2 #4 I think is the one that actually mounts direct but that limits you to I think a 165mm rotor.)
I'm Definitely leaving the right hand side for someone else.
Anyway here are the pictures of my LEFT side +43 mount for the dual disc mount Marzocchi Z1 1997 that I already know isn't the final version.
Sorry the photos are sideways and I have no idea why.