This is a photo of the chamber after having the larger valve seats cut. If you look closely you can see that the bowl looks like an hourglass, narrowing below the seat and opening into the bowl. It's particularly pronounced to the left side of the intake and the right side of the exhaust in this photo. This will have to be opened and blended to allow the air to flow through without interference.
After dropping off the heads at the machine shop I was told to return in two days to pick them up. But, they weren't going to be finished, they were only going to have the new valve seats cut-in. The plan was to get them back after the machinist had cut the larger valve seats and blend the valve seats into the bowls myself.
When the day came to pick them up the machinist showed me the drastic narrowing of the bowl area. The narrowing looked like an hourglass below the valve seat and above the bowl area. From this it appeared that even before the cutting of the larger seats that the bowl was restrictive and could have used some help. The machinist thought that this was to help low lift flow by encouraging the venturi effect through that narrow passage. He recommended that I not use a carbide burr but rather a cartridge roll to open them up since I would be working near the freshly cut valve seat. Also, I would have to make the bowl underneath the valve 85% of the valve size if I wanted the port to flow effectively.
I took the heads home and got to work. It became evident really quickly that the cartridge roll wasn't going to get the job done so I broke out the carbide burr and went to work, all the while holding my breath waiting for the almost certain slip up.
I'll detail the work done in the photos accompanying this text.
Right: here's a close up of an exhaust bowl before I started working on it. It's hard to see in the photos but the right side here cut back at a sharp angle from where you see the bottom edge of the cut.
Left: This is after the carbide burr and the cartridge roll cleanup. It's a different angle on the same bowl but you and see the larger diameter fairly clearly. Also look at the edge of the intake port visible and note the difference in materiel visible in both pictures. I didn't have any tools to measure the bowl to 85% of 1.6" so I just swagged it and attempted to make all the bowls the same.