Stock exhaust manifolds vs headers

My vote is to get a stock flange gasket and use it to compare to the size of the original style manifold and take a die grinder and port the manifold to match the size of the outlet gasket,
When I did mine I couldn't believe both the extent to which the cast hole in the manifold was 1)smaller than the outlet gasket and
2) off center relative to the studs.
I bolted the gasket on, and took a die grinder and "went to town". At one point the original cast hole touched the edge of the gasket and opposite I took out about 5/16-3/8" of cast iron to even out the exhaust hole.
And tapered down from that to nothing, where the flange gasket touched the existing port hole. I basically had to grind material around about 3/4 of the circumference of the exhaust flange to accomplish this.
The hole in the flange gasket I used is 2-1/4 diameter.
Which is a super common size of upgrade for a /6 exhaust system.
When you dump 1-7/8 worth of exhaust into a 2-1/4 pipe that is a pretty stout restriction/choke point. But opened up to match the head pipe will be a noticeable improvement.
RRR had a thread on doing just this as well
And there's plenty of meat left in the manifold when done
I know that/6 manifolds were very prone to cracking. I wonder if this bottleneck to flow may have contributed to that tendency back in the day...