When it comes to waste piping pitch, there are the rules, and there is observed reality. I have plumbed dozens of new homes, and done lots of repair and renovation. In that time I have seen multiple examples of pipe installations that, according to the rules, should be nothing but a clogged mess, yet function flawlessly for years. Particularly, if its ABS, or PVC pipe, which apparently is slippery enough, that it works when logic tells you it simply can't.
On several occasions I have seen long horizontal runs of pipe heading off to a fixture with no support other that the fact that it's hanging off a trap under a sink. Over the years the vertical run to the trap slides down, and the run develops a noticeable negative pitch. I have even asked a homeowner if the sink ever clogs? never heard a yes.
At some point, decades ago, my house was totally re-plumbed with plastic drainage. The installation is a joke. The toilet had a horizontal run with a 3/4" negative slope for a few feet. The sink has a 15' run of 1-1/2" in the floor joists that is unsupported. It sags down dramatically in several places, and by all the rules, should never work. It's trouble free. There is a ten foot section of the main 3" run that is pitched at 15-20* or so. I just bought the place, but I'm not surprised in the least that there are no drainage issues.
In your case, you are replacing a cast soil line that wan't installed "by the book" almost a century ago, yet performed just fine. You are replacing it with a product that is far smoother inside, and far less likely to develop obstructions as it ages. There is no reason to do the extra work of "correcting" something that works just fine. Pull the old stuff out, glue the new stuff together, connect it to the cast with a Fernco coupling, and you're done. Once you get this one done, you will be a bit shocked at how easy it actually is. Good luck.