340 coolant in oil

furrystump said
Make sure the fire rings are still round. La’s tend to push the head gaskets into the valley. That's why on the r blocks they have a provision for a fifth head bolt.

I keep looking at the OP's pix and they look round.. But the view is so far away, that it is hard to tell. OP, it might help if you could post close ups of the head gasket, perhaps 1 of the front 2 cylinders and another of the back 2 cylinders.

Also, could you post a pix of the bottom side of that head that you removed?

Garrett Ellison said
What kind of intake does it have? It’s rare, but I have seen an intake crack through the exhaust cross over into the plenum under carb and cause what you described. It may merit a pressure test on exhaust heat crossover passage in manifold as well...

Which points out that it could be quite a number of issues.