The Cleveland iron head has a cast in "restrictor ring". It is intended to increase the velocity through the pockets.
When "performance" prepping the heads, that is commonly cut out with the 60° pocket cut and sometimes the iron valve guide is also and replaced with a pressed in bronze guide like the aluminum heads use.
There is easily 25-30hp to pick up with those modifications but are intended for manual transmissions and performance driving. Not automatic transmission cars used for everyday transportation.
There is a reason a Boss 351 is only 330hp. It was detuned in many ways to make it a dependable street car. The valve pockets is one area.
You also need to narrow the seats by about half from stock but remember that is for racing and those engines get torn down al ot so cleaning the seats isn't that much of a big deal.
If you wanted to put 100,000 miles on the car before you do the valves again, you should stay stock.
The only thing that I would add to that is likely the stock iron valve guides only last about 30,000 miles.