Jim, A really convenient flat surface for checking purposes is your nearest window! With the lower studs removed any high spots and rock will show up. To endorse RD, a quick rub down on a piece of glass supporting the abrasive paper was the time honoured way of achieving a flat mating surface.
With the valve guides removed your machinist can easily check the head face and take a light skim as necessary.
Used rocker boxes always carry the risk of pulled, worn and weak threads, plus the prospect of the unexpected crack you can't see in that well posed ebay picture. There are still plenty of them around, from the neglected and relatively unworn barn find covered in grease, to the shiny polished and so correspondingly expensive offerings.
For me, it's what's inside that counts and for that price level it has to be something special. Unless you have a definite area of damage or distortion rendering your existing rockerbox unserviceable, making the existing casting oil tight should be an easy call. Worth checking all the hold down bolts are tightening the box down, and not bottoming in blind holes. Corner studs also need to be fully into the casting, and the retaining nuts not bottoming on the plain area of the studs.
A set of aftermarket mushroom head tappets will help with the slight misalignment with the valve stems. Usual practice is not to bore cylinders until pistons are to hand, the bore can then be sized on the basis of the running clearance calculated from the true piston dimension.
Swarfy.
