Thanks chaps, somehow I feel almost a sense of duty not to screw this one up but I've never been a slave to either period correctness, or, indeed, the rhythm.
The Powerlites are at the local framebuilder getting the steerer extended. They are in cracking condition so should look just so. Pragmatically with it being a 1 1/8th steerer I'm going non period with ahead (easier to get a headset plus I have a NOS, 1 1/8th zero degree 150mm black Syncros stem that will do good duty up front).
Following that, I completely agree that M735/6 is the way to go although I've got some low profile XT cantis that are slightly newer (for the life of me I can't find an M number on them though).
I'm messing it up already, aren't I?