70-72 A body. disc vs drum lower ball joint

I`m looking at swaping a 10 drum setup for a kelsey hayes disc set up . What is the difference in the lower ball joints {I dont have the disc setup only E mailing to the guy that has the setup for sale } . He says the lowers are the same, I say no ,based on Rock auto and Moog sites ??? I sent him a picutre of mine he says they are identical. Are they that close?? The bottom bolt holes on spindle are 3 3/4 " a part and the lenght of steering arm seems to be 4 3/4 in lenght. The lower drums are as much as $125 on some sites where as the disc lower are alot 1/2 as much... must be some difference ??? HELP

They are not the same, I have both setups and the moog and rock auto sites don't lie ... in this case .

The difference between the two,is the mounting bolt hole diameter to the spindle.
1/2 inch diameter for the Drum brake ball joint.
9/16 inch diameter for the Disc brake ball joint.

Soooo if I really had to I could enlarge the hole in the drums ball to use my new balljoints on the disc spindle .... nothing else ??. Thanks

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Soooo if I really had to I could enlarge the hole in the drums ball to use my new balljoints on the disc spindle .... nothing else ??. Thanks

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Yep, that's correct.
Just drill them out to 9/16 diameter, and your good to go.