I put disc brakes on my '65 F100 and used the complete I-beam set up from a 73 pickup. I would think you could swap just the spindle as well. Another thing about getting everything off a donor truck, I got the proportioning valve and everything I would need to do the swap.
You can get drop spindles. Pick up a Truck magazine and you'll see ads in there for companies that supply them. Fat Man Fabrications comes to mind for some reason, I don't know if they'd have them.
[also moving this to the suspension forum

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