engine rebuild problem: valve "slap"?

I have seals that came with my gasket kit, so I'm definitely using those. And yes the labor quote included bench shimming the valves.

so I will undoubtedly need new shims after I get my valves done, right? But I won't know what size until it's all said and done, correct?

Your head should be real close when it comes back.
Bolt it up. check the shims, if close run it. Run motor up to norm. operating temp.
and shut it off. I what 12 to 24 Hr. and check the shims again.
After that you should be good to go.