I agree with what everyone says so far. Practice, practice, practice. You really need to get a handle the die grinder first or you can hack up the valve seats and guide bosses QUICKLY.
I'm attempting to repair a hacked up set of 291 heads. They'll need thick walled guides pressed in to repair the damage to the guide bosses and probably won't do much more for the engine than they would've in stock form.
Too much metal removed in the bowl area created a low pressure area, see the circled area. A hump behind what remained of the guide boss further upset the flow, see the arrows. The picture on the right is of the same bowl after the port cieling was raised and the hump removed.
This pic shows how far the port ceiling had to be raised above the already raised ceiling to eliminate the low spot in the bowl.
Here's one of them ready for the machine shop for a cleaning, new guides, a valve job and resurfacing.
Take baby steps when you're removing material or you'll end up with a mess like this. It's going to cost this guy a bit over $600 for the work I'm doing plus the new guides.
The pushrod pinch is at the port entrance right next to where the pushrod passes through the head.
Welcome to the world of cast iron boogers.