OK, so.
In real life, the Chicago White Stockings aka Cubs played most of the 1871 season at the Union Base Ball Grounds (aka a few other things) until the Great Fire destroyed it and made them a road team. The White Stockings then went on hiatus 1872-1873 before rejoining the National Association in 1874. The 23rd Street Grounds was built in 1872, but since the White Stockings were on hiatus, they didn't start playing there until 1877. So, it makes sense for 23rd Street Grounds to be the Chicago home park starting in 1872 if you have that team playing, especially since the Union Base Ball Grounds were destroyed.
The 1871 park was on the same site as the later Lake Front Park, used 1878-1884. In kicking around various possibilities, I think a Union Base Ball Grounds model will look a whole lot like the existing Lake Front Park model, given how little is known. I could just take Boston's 1871 grandstand and put it in place of the other grandstand, but after discussing with Garlon I think what makes the most sense is just using the existing Lake Front Park model in 1871 in addition to the later years. Having said that, I hate disappointing people, so if there's a hue and a cry I could revisit this (or another modder can!).
So, I think that leaves the Capitoline Grounds and Newington Park, both of which I think I have enough information for to make modeling worth a go. I also started on Greenlee Field, so hopefully I'll be able to post that in the other thread before too long!