what whould be a fair price to have a reputable mechanic replace the valve seals or o rings in a 84 Z28 with a 305? Can it be done with heads left on the engine?

You really ought to consider doing it yourself. It's so simple.
Unfortunately it's time consuming, so I wouldn't expect anybody doing mechanic work for a living, to do it for less than $150, probably more like $200.
Let it smoke for a few weeks while you study up on what it takes to do it.
If you own one of these cars, it would be a good idea to know how to set the valve lash, etc. anyway.