The only advice I can really give you is to just read as much as you can and go to work.
Wikipedia, protoball.org, 19cbaseball.com, and the
SABR 19th Century Research Committee are sources I've used. There was a fifth that was hosted by Major League Baseball but that website has since been taken down, unfortunately.
Another thing to understand is that you're not going to be able to perfectly recreate pre-1871 baseball in OOTP. Games then only involved one pitcher per team no matter the score, and that's just not a thing that will happen in OOTP. I experimented with limiting teams to zero or reliever in the "Stats & AI" settings, but what would happen in high-scoring games was that teams would start throwing in position players as relief pitchers. From what I can gather, there seems to be a hard minimum of four pitchers per team per game in high-scoring contests.
Another thing is that the game is built to sim baseball from a more modern environment, so even if you have the settings down to what you think are correct for the time, you'll likely end up with runs per game figures lower than what they were in real life.
Long story short, have fun with it and don't try to make it perfect.