I just did one of these today. Like Mike & Burnt said, get rid of the radius on the breach bolster. I think most guys just bend to fit, trial and error, no need to make a big deal out of it, but here is how I do it.
First get rid of the radius
I use one of these contour gauges. I set the barrel in without the breach plug, put a mark where the breach is located and take the barrel/wrist profile.
Set the tang in the vise. I eyeball the centerline of the barrel with a square
Bend until it fit the contour gauge profile