A 71's not that old!
Are we talking front or rear? You'll need to check the thickness of the drum if it's below the spec's stamped on them, then yes you have to replace the drums. If the drums are ok, them you may just need new shoes, and a wheel cylinder rebuild kit!
There are several makers of front and rear drum to disk conversions, price ranging from $700 to $2000+, but you get what you pay for, some kits include everything, new spindles, bearings, MC's lines etc! while some are bare bones.
If you are good at scrounging parts and turning a wrench then try scarebird's,
http://www.scarebird.com/ They are the cheapest, basically they supply the custom brackets, you go to NAPA with a shopping list and build it yourself. I'm going the scarebird route on the fronts of my 64' over the winter when it's off the road, and I can scrounge parts on the cheap!
Other makers include Master Brakes, SSC among others, just type plymouth disk brakes in the google bar and you'll get all you need!