Dana 60 S60 or Ford 9"

At that power level you might want to get some professional advice. My guess is that the advice you get will be to go with a fab housing and a 9 inch or even 9.5 inch Ford type rear end. I'm not sure where the power limit is on the S60 but you don't find them under big power cars. I think it might have to do with the fact that you can fabricate a brace for a 9 inch but you can't really brace across the back of the Dana 60. That might be what your chassis builder was trying to tell you.