Actually it owuld proabably be cheaper and easier making your own conversion kit. Making a moutning braket for the calliper is realtively easy from there you just need find a stock rotor that will fit your spindle once you find the right part number get those parts from a slavage yard you can pick um cheap then get new rebulit ones from the auto parts store. The reason you get the old ones that may or may not be any good is for mock up and they will save you a ton on core chargers when your ready to put the new stuff on.
Disk brake conversion are not diffcult at all. Unless you want baer 6 caliper with 13" cross drilled and slotted rotors. then you can get up there. but for a driver espically a 55 pontiac. (I am assuming your not running any 20" + open wheel designs) its the perfect way to go. If your worried about weight check into pick up calipers and rotors.
BUt like my freinds suburban it cost him 35 bucks a caliper for rebulit ones at the part store but the core charge was 50 bucks. so that an option you might want to look into.