Hard to say about the brake calipers. The "brake shoe gap" between 27" (630 mm) and 700c (622 mm) is only 4 mm
((630 - 622)/2 = 4 mm), and I think I see some gap in the photo.
I've never done this with Weinmann 500 sidepulls, but I have with Shimano
600-6207 on my 1984 Trek 610. The Trek
was well-designed to accommodate such a conversion. Slight reduction in deceleration, but my hands strengthened
pretty quickly. I was acclimated to it pretty quickly. But still, I had the 500's on a 1969 Italian bike with a lot of early '60s
parts. I always thought the 500's were not strong, in terms of "stop me now with no hand pressure!" With a good or great
hard squeeze I could stop quickly. I never really saw the benefit of say, modern dual-pivots, until I installed Campagnolo
dual-pivots on my Mondonico. THAT was a great improvement over the short Shimano
600-6207 that the bike came to
me with!
If the Weinmann brake caliper reaches, I would make the conversion to 700c if there is a need for tubulars. Otherwise I
would stick to the 27 x 1 ¼ rims. I'm replacing rims on my 27 x 1 ¼ Rudge, and with admittedly heavy Wolber Modele 58
and Pasela Protite wire-bead rims, I think it is going to work well.
If you have a set of spare 700c or tubular wheels, do a test installation. That will tell you all you need to know in real-life
terms.