might be easier, and less expensive to buy a clincher wheelset that's compatible with the droput width of the bike.
Rebuilding is as costly, or more, than just geting a new set. There are plenty of options - even new stuff - on ebay for very serviceable and nice running clincher wheels. If the old wheels aren;t Campy, Dura ace, Suntour Comp or some high end vintage stuff like Phil Wood, then they're not worth converting. The vintage forum might have good sourcing for getting appropriate wheels for the level of the bike in question.