Man ball joints are really expensive, and I know you have made up your mind
BUT I hope you dont end up regretting the decision to keep drum brakes especialy on a parts runner that will be doing a ton of in town stop and go driving and probably some towing, you may have terrible trouble with fading and stopping under heavy traffic conditions in the summer when the heat is really a problem. I mean its only money right, I suppose you can always shell out the money for the 1971 Ball joints later it just seems like a really big waste of money in the long run when you can easily get all the original 1971 GM parts you need to convert it over to Disc really really darned cheap, just sayin...
Edit: Just incase your response would be "I already have the ball joints from another project so I didnt have to spend money recently for them specificaly for this truck" Then my response is ok then fair enough.
