I've bought some Willwood calipers to replace the Sierra units on my Fury but now understand the Willwood's require an imperial connection
on account of being US sourced. My current flexibles are metric connecting to a flared pipe and metric connector.
What's the best solution; is it possible to get an adaptor or is it best to have new hoses made up with metric at one end and imperial at the other?
Not much help but my kit came with Willwoods and they had adapters so they are available. Have you spoken to Rally Design?
The Willwoods have a 27 x 1/8 npt thread, you can get a male male adapter for them
but I assume your existing hoses are male ended so you would need to swap the existing ends for female 3/8 x 24 JIC to match the adapters, as long as you have about 20mm or so spare on the hose length (you would gain a bit back anyway as your female will be going on the outside of the caliper rather than going into it) then you can just get the ends changed (assuming they are proper flexies not OEM rubber things)
Drop Ollie an email at email@example.com he should be able to provide adapters & change the hose ends for you