The current replacement ball joints are affordable, but back when they were $50 each and still needed grease fittings, I modified some Chrysler-type (NOT made by Chrysler) upper front ball joints to fit our Pantera. These are fine-threaded, designed to screw into an a-arm socket although I didn't adapt them that way. They came with boots and zerks.
This part is quite popular with kit-car builders for big-block Cobras, kit-GT-40s and dirt track race cars. Some have replaceable tapered studs of different lengths for changing the geometry of the suspension. I used only upper fronts for all 6 ball joints in the car. They work fine and I managed to lose some unsprung weight in the process. Illustrated article in the POCA Archives