Oil Pan and gaskets 1st Gen SB

[PbFut]

Sorry this got a little long.
I have had sealing issues on the rear main and pan since I bought the truck a year ago. The leak was pretty bad so right off I pulled the pan to swap the rear main seal. Believe it or not the upper section of the 2 piece seal was missing. That did not leave a feeling of confidence in my new purchase. To be on the safe side, I used a FelPro offset RMS in case the crank was scored. I had a heck of a time getting the correct oil pan gasket. The gaskets I received seemed too short in the rear arch of the pan that was on the motor where it mates to the rear cap. I found a FelPro 1885 and it fits the pan perfect so I thought all was good. The area stayed dry for about 4 months. Then I pulled the motor to do a deep clean and new suspension. I decided to do a top end refresh. After a cam challenge all turned out well. A few weeks later I start getting a drip again in the RMS area. Pulled the pan and to me the area around the main seal looked dryer than the pan area. Swapped in a new seal and then started thinking about the trouble I had with gasket last time. So I ordered a simple Dorman pan part number 264-104 off Amazon and a new 1885 gasket the other day. The 1885 gasket fits my old pan but not the new Dorman pan. It’s obvious the new pan requires one of those other gaskets that did not fit the old pan last time. Given the missing upper section of the rear main, I am thinking this old pan and 1885 gasket is designed for a different configuration. Is there a rear cap with a slightly larger radius that mates to the oil pan. I dry fit the new Dorman pan without a gasket and it seems to fit correctly with a very slight rock. Not nearly as much as the space the 1885 gasket has to the new pan. I am now thinking this recent leak is due to an incorrect pan and gasket but I cannot find anything relative to a pan having a larger radius in the rear. I looked up the 1885 part and there is no reference I found relative to the rear cap/seal area. Obviously this old pan has a larger radius as does the 1885 gasket because the gasket for sure does not fit the new Dorman pan and its clear those gaskets I had from months before would fit. But looking up a FelPro 1885 it says 1st gen 2 piece seal.
Sorry for such a long winded question post/post. I am sure this is a simple answer for someone here. Given the engine was missing the upper half of the rear main seal when I got it, I now think the pan is wrong as well and the new Dorman part correct. If I get the correct matching gasket, I think this leak journey will finally be over. But maybe for future reference to others, someone can step up and answer what this 1885 gasket and old pan I have is for.