Is it really necessary to modify the crossmember for a dual exhaust?

When my Dad & I put mine together back in the winter of '70-'71, he used the complete exhaust system from the '68 'cuda the 340 came out of, manifolds, hangers, and all. Notched the rear valance to clear the tips. I don't recall ever having a problem with it hitting. Pretty sure I would have noticed as I hated the rattle from the heat riser until I tied it open with a coat hanger. Dad wasn't too thrilled with that. :lol: