Vintage Peugeot conversion help and tips please

well i'm currently running on two chain rings, and wanna move down to just one, as i'm on 52 teeth right now, and want something lower!

also, i recently fitted a shimano br-r450 brake, but it's still not low enough, and the pads are slightly off the rims. is there anything that can be done about this?

has it got bolts that hold the chainrings on or is it stuck together with two? about the brake, maybe need to get a different one with longer reach. have you put a 700 wheel into a frame that takes 27" wheels?