I'm converting a 9" drum system to a disc brake set-up. I've got the upper control arms,spindles roteors and calipers from a '74 A body. Do I need to swap out the lower control arms or use a different set of lower ball joints on the original l-arms?

The lca is the same & you can drill out the 2 mounting holes in the lower ball joint as the thru bolts are larger dia. There is plenty of material to do this & the metal is soft, your drill press will do in in a matter of minutes.

Thanks! That's the ticket!