Exhaust port/hole outlet size and exhaust pipe diameter off manifold to muffler

Looking for a measurement for the exhaust port size and correct pipe size to use coming off the stock exhaust manifold /header- 1966 Chevy G10 Van 6cyl. 230.
Anyone know what the correct diameter(s) for the pipe coming from the engine manifold mount to the muffler? *I guess that depends on the muffler to an extent* but what was the stock set up?
I can tell that what's on there right now does not look stock or correct. Also since the exhaust is all welded together, I would have to take everything off to get that measurement on the port from the header.
I can also see a reducer or shim ring behind the mounting point at the header from the exhaust pipe to the muffler. The muffler seems undersized and is an auto-store generic replacement muffler. It has blown out on the seam a few times. I believe this because the exhaust pipe from the engine is too small and increasing pressure on that small muffler. I patched/repaired the blown out muffler in the past and did not have as many cold start/idle backfires but still there was a small leak making it prone to backfire and causing it to blow apart again.

So I just went ahead and took off the exhaust pipes/ muffler. Looks like the shim/reducer ring I mentioned is a donut gasket and also the wrong size - much too big and it was installed crooked. This created a big hole in the side of it which may have also been contributing to the backfires when cold. The pipe where it connects to the exhaust manifold measures about 2 inches in diameter and was cut/ spliced after the bends to reduce down to a 1 3/4 into the generic muffler.
The hole on the exhaust manifold/ header looks about 2" as well. Can anyone confirm that?
Also, does anyone have any pics of the correct type of gasket to use between the exhaust manifold and the pipe?