340 build .30 over

From what i got from shop. in order to make my rockers sit correct on the cap i would need to change my valves and possible head work around the valve seat.. heads are already together. I dont think it will be a problem if im not reving it like a nascar..wish me luck.

Why would anyone do all that head work when a simple rocker change would fix the geometry, and cost less? Im confused that a shop would even recommend doing so.