Cheap way to convert my 1968 chevy C10 to 5 lug rear drum or disc

[MDPotter]

I would say it depends on the condition of the axle you have and what your wheel/tire plans are.

The issue with this year of trucks is that the axles are narrow. The rear track is 1.5" narrower than the front. And if you're doing a disc conversion, that increases to a 3" difference. I have a Dana 44 and can't find 5 lug axles for it, so I am going to use wheel adapters/spacers. Wheel Adapters USA makes an adapter/spacer which converts to 5x5. It is 2" thick, so you will probably have to find wheels with a fairly deep backspace. I found a 9.5" wide rear wheel with a 5.7" backspace which should work. The adapters are $115 for the pair.

But if you don't care about the track width issue, it comes down to the condition of your axle in my opinion. If you have a 12 bolt that is in good shape, I would go with the axles from Summit that Jason linked. And by good shape, I mean it is painted, brakes rebuilt, new bearings, seals, and no known issues. You never know the condition of an axle in a junkyard and it will require a lot of work to bring it up to the condition of an axle that you've already put a lot of work into.