What to check after a few hundred miles on a rebuilt engine?

I can't imagine why you would need to retorque head bolts for an oil leak. Do you mean valve cover fasteners? If so, yes you can try that, sometimes the gaskets can shrink from disuse and fasteners become loose. But I'm thinking it may already be too late for that. It takes a light touch on stamped steel v. covers. If the cover rail has become distorted from previous over tightening, you'll have to straighten it first, then replace the gasket.