What carb/intake are you using? You don't happen to have one of the 2 piece welded intakes do you?

Was the head or block decked much? If so the intake may not be sealing against the head. When you deck the block there is a casting block that will hold the intake away from the head.

From that pic with the head off it looks pig rich. I have seen that caused by vacuum leaks before.

Carb is a Holley 1945. Intake is stock. Deck and head were milled just enough to make it flat. Probably less than 20 thou. It is running really rich but mostly due to the amount of cold starts. It hasn't been warmed up properly for a while. I'm leaning towards a vacuum leak being the issue but I'll change the carb again and see if I get better results.

Just a thought, but did you check for TDC? Maybe the crank gear key sheared.

Fouled plugs don't "unfoul" themselves, and they also create carbon hotspots which screw up cylinder firing. so if you are doing all of this diagnostic work with fouled plugs, you're wasting your time

the other thing I notice is the exhaust pipe flange. it looks like the entire thing has been leaking. slant sixes are very sensitive to exhaust leaks because they are small engines and already dialed in to be very lean. that could certainly cause popping.

Make sure the heat riser valve in the exhaust manifold isn't frozen shut. that'd make it run like a dog

Installed a brand new set of plugs 2 days ago. Exhaust is only tack welded together so a leak at the manifold flange shouldn't have changed how it runs just recently. The riser is free and even pulled the EGR and cleaned it.

You have fuel, compression on all 6, spark, it should run if the fuel and spark are right. You have no vacuum leaks. Compression rules out bad cam lobes, valves, timing chains and head gaskets. Two different carbs rule out fuel, unless they are both bad.
Must be an ignition problem of when it ignites. Is the mechanical advance working? Try timing it by ear instead of a light. Could be a bad light or slipped damper ring giving a bad reading.

My bet is on the valve lash. Maybe an exhaust seat is pounded out or if too much material was taken off of the head ?

The head is off and I can see the valves and seats. They look good. Can't get it to run well enough to even tune it by ear. Like I say it has me stumped. The compression test should have shown a bad valve. I'll give it another carb change and see if that does it when I get the head back on.

I doubt if you have one laying around but I'd wanna swap the intake manifolds. After all this I would have to know what it was all along no matter what the cost

You don't know the kind of pack rat I am. I do have another intake laying around and I think I'll try it when I put the head back on. That is one thing I haven't tried yet.

Installed the new head gasket and blocked off the EGR port. It runs like a champ again. Was it the head gasket? Was it the EGR? I don't know but it runs good again. Feel much better about it. The head gasket looked good enough to reuse so I don't think that was it but the new one I torqued tighter than the specs ask for thinking it might have been leaking across.

If the EGR was stuck open it would run like crap.

Glad to hear all is well again.

Very well could have been the problem all along. I hate anything to do with emissions. It certainly caused me several days of extra work and head scratching. It is nice to be able to move it around under of it's own power again.

Success! Just me I'd undo the EGR & see if it acts up just like it did before . You could do it & reverse it in no time. After all this I would HAVE to know for sure

I've got a bunch of work to get caught up in the shop first but I probably will try that to settle it in my mind as well. I still have to put all the things I took apart on the darn thing back like they should be before I can drive it out to clean up my mess. This delayed the tranny rebuild by a month and I had hoped to have the interior painted but it is good to have it running again. My son will be driving this spring and I plan on having it completed so he can drive this. I do think it was a good learning tool for him also. We did a lot of trouble shooting that is not normally done.

That is one of the things I keep telling him. He needs to know the basics on how it all works instead of just plugging it into a machine and replace the parts that it tell you to. Anyone can change parts but fewer guys anymore can work their way through the process of finding what's wrong the old way. Under all the electronics is still a mechanical base.

I had forgotten all about that EGR valve on the outside (pass) side of the ex manifold. I swapped 1 /6 30 years ago & hit the EGR valve during the install & partly unseated it & from my verbal descrip of how it was acting a chrysler dealership mechanic immediately knew what it was