can anyone advise me on what is the cheapest way to convert my 72 C10 to disc brakes on the rear...  

Later Camero will work with some removal of unwanted brackets. Also Lincoln Visalle has a narrowed 9" disc, there cheap to buy the car because most people do not know what the rearend is. But if your not good at fabricating it could cost you more than you think. Sometimes buying the conversion kit is cheaper than changing brackets and/or stub patterns.