Rebuild on a 455 costs around $1200 here...heads pocket ported, crank turned, TRW pistons, rods balanced, new cam, etc. As always, your mileage may vary.
On your guy, ask for references...anybody he's rebuilt Olds engines for? If he says no, just nod and smile and move along.
Either way, go to
www.oldsclub.org and look for your local (or closest) chapter. They can be a wealth of information on an "Olds man" your area. And joining OCA is some of the best money I've spent on my car.
C.J.