Right then,sorry to be a pain but Ive tried a 26 wheel in front and back and the results are
front leaves 85mm from centre of brake bridge to rim
rear leaves 90mm
Ok. Look on this page: http://sheldonbrown.com/rim-sizing.html
A modern 26" MTB wheel is 559mm
Old 26" wheel is 597mm (if I've picked the right ones). Which sounds about right.
A 700c is 622mm.
A 27" is 630mm
so with the old 26" wheel your brake drop would have been
front 66mm, rear 71mm.
(worked out drop by difference in wheel size compared to 26" fitted divided by 2, would be good if someone else could verify those measurements)
my above measurements are right then I reckon it was built with 26" wheels as they tended to have a relaxed geometry and 27" would have made it quite tight.
my measurements are right then I'd go for 700c with some long reach brakes 47mm - 60mm'ish which are quite widely available and not unsightly. Or 27" with regular brakes...
Not really comfortable with you making a decision solely on my calculations or suggestions though!