Looking good. For reference images, have you thought of checking out the brass modelling scene? Those guys are fanatical about detail, and often have access to original factory blueprints to work from.
Just as one example (this is the same type as 1225): http://www.brasstrains.com/Classic/Product/Detail/064119/S-Scale-Brass-RRM-River-Raisin-Models-Boo-Rim-PM-Pere-Marquette-2-8-4-N-1-Berkshire-Ptd-Unlettered
The images are downloadable as 1600x1200.
One detail to note, and this applies to most steam locomotives, is that the driving wheel counterweights are not all directly opposite the crankpins. They are at subtly different angles.
ETA: The Nickel Plate Berkshires of the period were almost identical to the Pere Marquette N-1. Drawings and pictures of those may be useful too.
Here’s a bit more. The Project 1225 site has a couple of detail photos buried in its archives: http://www.project1225.org/
Also, the background image for the site is an original, large scale, factory drawing of the firebox.
There are also a lot of detail photos here, if you have a Pinterest account: https://www.pinterest.com/ces316/pere-marquette-1225/
You could also ask the current owners of the locomotive if they could help you out: http://michigansteamtrain.com/