Got it done today. Not quite as bad as I expected. I had to remove the steering stabilizer to get the oil pan out, but that was just 8 bolts. There isn't much room to get the oil pan out! It barely fits out and that was after having to unbolt the oil pump pickup tube and let it drop in the pan to buy a few more inches.
The existing rear main was a 2-piece rubber seal. I was having a hard time getting the top one out. I tried a welding rod and a punch to get it started and it wasn't moving much. I then had the idea to take the old lower seal and flatten it out with a hammer. Since it's the exact same profile as the upper half and the wire inside is the same, it mated with up with the top seal. I tapped it a couple of times with a hammer and it started working its way around. I cleaned everything up real good, installed the new seals and put everything back together. The oil pan gasket I bought was a 1-piece rubber one, so it was relatively easy to get back on, too.
I have never looking inside the bottom end of this engine before. When my original 223 blew, I found this one locally and was told it was in good shape so I just dropped it in. There was a tiny bit of sludge in the bottom of the oil pan, no metal shavings at all and everything looked surprisingly clean. The rear main that I took off and the oil pan gasket was rubber, so some kind of work was done to this thing at some point in the past...but it was most likely a while ago.
I am going to let it sit overnight and have the adhesive on the oil pan gasket cure before I pour in the oil and fire it up. All said, I had about 5 hours in the project. Not too bad. Now, I just have to cross my fingers and hope nothing leaks tomorrow!
I will post an update after I take her on a test drive tomorrow.