Caliper upgrade???

I'm getting my 73 dart up to "my wife being ok taking our son in the car" level, but I know nothing about brakes. Well not nothing, because I know my front brakes aren't working. Originally I was just planning replacing the rotors, calipers, and pads, but now I'm wondering about upgrading. So here's the question: what is involved with upgrading the calipers to something bigger?? is there something that bolts on, or do I need an entire kit??

Thanks as always