No argument with what George and Gary have said, however; and this is only on the basis of what I have read here on the BB; the old Volvo valve seats, although they didn't have hard inserts, were case hardened to some extent. Furthermore, over years of use with leaded gas, they would harden some more, or, at least, acquire properties that made them act as if they were hardened. So, if the seats have never been ground or lapped, it is likely that you can use unleaded gas and not have them recede. I would say, run the thing with unleaded and use a lead additive (for the real or the placebo effect), but keep an eye out for drastic valve gap reduction.
Bob S.