Slant 6 Vibration

With the engine overheating, your head could be warped, so I would certainly take the head to a shop to be checked for flatness. That warpage could cause a head gasket leak; those 2 things, warpage and head gasket leaks, often feed each other.

It aaaaalmost looks like there is some leakage between 1 and 2, but it is hard to tell from just a pix. And it looks like the normal rust-through spots in the head gasket have occurred, which lets some of the coolant bypass some of the block and raises engine temps. So it is good to get that old metal shim gasket replaced.

And again hard to tell, but it almost looks like #2 exhaust is a bit open (...which would be odd, so it is probably just the pix). Check all of your valve springs to see if any are broken. As said, you cannot really see if you have a bad valve seat until the valves are pulled. So either buy/borrow the tool to remove the valves or take it to a shop to examine. If you get the springs off, then this is the time to put in new valve stem seals; the old ones are no doubt shot. Cheap....

The inside looks VERY good! And good on the cylinder bores. Just for education, too shiney is actually not good; that can be what is called cylinder glazing and the ring seals fail. But if the compression did not come up with oil in the #2, then that is not likely any issue for you. Since the head is off, I would borrow the tools to accurately measure the bore diameters; not essential to do, but now is the time to find out. (Measure them across the long axis of the engine.)

When you take the head to the shop, they will likely find:
- perhaps some head warpage
- very possibly a burnt valve or seat in #2
- loose valve guides
If you are going to keep the /6, then now is the time to do it all: head mill, valve guides, and valve job. That all is gonna run a few hundred bucks, BTW. If you are not going to keep the /6 long term, then skip the guides, but you'll need to get any head warpage milled out, and any burnt valves/seats re-worked.

And know that the newer head gaskets are thicker and thus will lower the compression ratio a tick, and make the eninge a bit less torquey. If the head is going to be milled to remove any warpage, just go ahead and mill it .060" and you'll end up with better compression and a bit more low RPM torque. You will indeed feel it.

While you have the head out for work, remove the lower block plug ( under the oil pump) and see if there is a lot of rust in there. I agree that some further efforts at rust removal are in order, based on what can be seen in the block, your rusted core plug, and the rust-through spots in the head gasket.

No certain test that you can run for a cracked piston at this point. That seems very unlikely to me anyway. What you can do:
- Inspect the tops and edge very closely
- Bring each piston up near the top of the bore and alternately push on the left and right sides (looking from the front of the car); the piston should rock very slightly back and forth to when you do this. They all ought to rock back and forth about an equal amount. This is looking for any severe breakage in a piston skirt (down in the bore) but it won't find any slight cracks. But cracked pistons are not common anyway, so I would not lose any sleep over it.