I have 26 year old shimano 600 brakes on 27" wheels. The pads are straight and about 1" long. I'm replacing the pads and wondered if I can use (assuming I find ones that will mount on the brake arms) the much longer pads I've seen in the bike shop? They look to be about 2+ " long with a curve and I'm guessing would have been made for 700 mm wheels?
Sounds like you are describing a brake shoe/pad assembly that is typically found on v-brakes typically on a mtb or hybrid. If that is the case, they will have a stud coming out of the back of the pad/shoe with a nut and some washers. By playing around with the washers and spacers and adding the appropriate toe-in, they will probably work as well as the OEM stuff....maybe better. Give it a try!!!!
I think you should consider that V-brakes probably flex a lot less than those old sidepulls, so the long pad combined with a flexy brake could result in the pad getting pulled up and into the tire sidewall.
I've been using Kool Stop "v-brake" pads for a couple of years now with my old Dia-Compe centerpulls from 1983. A wonderful upgrade, they're not only better pads but they have spherical washers on the posts so toe-in is a breeze. My recommendation is to get Kool Stops in the salmon compound, they're the best brake pads out there in my opinion-
I'd say it depends on the sidepulls. I've got an old 70s schwinn suburban that I replaced the pads with some spanking new Kool stop V-brake pads and have no problems with flex... on the other hand I put the same pads on my girlfriend's 4 speed and her brakes have a bit more flex. She doesn't stop hard enough that it is a worry, but I wouldn't use that combination of pad/caliper on my setup.
Also, most decent V pads have some curve to them so that will help offset the extra length.