Pictures would help us provide an answer with less confusion.

I recently restored an older Peugeot mixte bike as a casual ride loaner and because I had all the stuff to put it together. I wanted to switch to sidepull calipers but in my case there was nothing I felt was worthy of withstanding the sort of load that a brake will generate. The only spot was a flat bit of bridging used to mount the fender. And that was far too flexible for my tastes to be used as a brake caliper mount. The caliper on my frame was sized such that I COULD have switched it over but I would have had to buy a longer reach sidepull. And since this was a full on parts bin "buy nothing" project I stuck with the center pull on the rear. With a switch to some decent brake pads performance is more than adequite for a cruiser/comfort bike such as these are.