“Social Security, like interest on Treasury bonds, and redemptions of maturing Treasury bonds, must be payed first out of any available funds during a debt ceiling conflict. And of course, changes to Social Security now will not affect the debt ceiling for decades, and are thus utterly irrelevant to the discussion here.”
One other technical note: the "IOU" mechanism would not seem to apply to Social Security. The Treasury can give the SSA an "IOU" and the SSA could theoretically give one to a beneficiary (maybe), but the Treasury cannot directly supply a beneficiary with an IOU, because of the trust fund mechanism. And, given what I noted before about the IOU proposal, that sounds for the best.