Hey boltsmissing -
You need to measure the reach of the frame to the new 26" rear rim. You do this by mounting the rear rim with or without a tire, and then measure from the bolt hole where the brake attaches, then to the center of the rim where the brake pad touches. This number, in mm, tells you what brake reach you'll require. It will no doubt be 'long reach'. I'm guessing from the type of frame it is, that it does not have cantilever mounts for v-brakes/cantilievers. You can weld those on and those types of brakes are far better and more powerfull than any side pulls. IMO. You don't really need that strong of a rear brake as most of your braking is done by the front wheel. Have fun.