[1969GPSJ]

I too have a 428 HO engine in my car, so I cut the exhaust pipes off close to the manifolds first.
Then I removed the engine (I was going to pull it anyway, but there ultimately wasn't a good way to get to the manifolds and bolts, etc. without pulling the engine.
After the engine was out and on the stand, I got all but 4 of the bolts to come loose, but those 4 rounded off to the point that I ended up carefully grinding off the heads of the bolts and after using a countersink bit on a drill to finish up and ensure no remaining bolt material was there to interfere with manifold removal, I removed the manifold from the heads, carefully tapping using a dead blow rubber mallet and very carefully prying too, after using good penetrating oil on the rust welds between the manifolds and the bolts.

After easing the manifolds off, I tried to remove the remaining parts of the bolts, but even with heat, 2 of them snapped off in the heads.

In your case, once you get the manifolds off of the heads, the ear on the manifold can be repaired pretty easily by a competent welder.

I have cleaned up my manifolds, but now I have to get the remainder of the old bolts out of the head.

To deal with the broken manifold bolts, there is a good way to do this using copper tubing and the welder, but that project has been on hold for a while. Hope my method for removing the manifolds helps you get them off of the engine without further damage.