In most parts of the country all it takes is a few calls to local machine shops to see what it would cost to refurbish and update a set of factory iron heads to be the equivalent of what you’d be getting with new aftermarket heads.
As in.......All new hardware, oversized valves, increased flow.
And unless you’re starting with 516’s or 915’s....... you still end up with the not particularly pump gas friendly open chambers.
Unless it’s an area of the country where the labor rates are uncharacteristicly low........ something like a pair of Stealths or E Streets usually end up being less expensive than reworking the factory heads.