I've doubled checked the c-c boss distance and it's 70mm for the fork (77mm for rear).
I've got the thinner spacing washer between brake arm and pad. With the Mavic A 719 rimmed wheel I'm trying to use there's very little space between the rim and inner fork leg for brake pad and its actuation. If I removed the washer altogether I can't see that would help.
I'm not actually sure if canti brakes would solve the issue, although it might improve the angle of the brake arms.
I was hoping to use 37mm pasela's, and whilst they fit (and I think there will just be clearance for mudguards) the brake pads won't clear them when fitting/removing wheel unless it's deflated.
I am lucky enough to have V and caliper brake levers in the spares box.
I'm beginning to think that my options might be a narrower rimmed wheel with max 32mm tyre and cantis (or possibly the V brake might then work) or the current wheel and 37mm tyre with a side pull brake. I do have a long reach tektro one which was for a 27" to 700 conversion and that might be worth trying. mmmm
And just to spice up the build a bit I've discovered that the fork ends are a touch out and need re-aligning too. Ahh the joys of bike building