Glad to see this update Roger. This is one heck of a big job, but it's coming along well. I can't wait to see the finished pictures.
I didn't quite follow you about the steel shims, so if they negate the need for this concern/idea, please forgive me. But I keep wondering whether body flex (e.g., driving ove railroad tracks) is going to break the wood shims loose over time - even with the pocket screw? IF it does, how much problem does it cause if the wood on either side of that broken glue joint can move up/down or back/forth?
Do you have space to reinforce it with some angle iron or by laminating more wood across that joint? If not, do you have room to wrap a wide automotive hose clamp around the shim to keep the wood on both sides aligned if the joint does break? I know that's ugly, especially on a beautiful full up restoration like this. But if a broken glue joint can cause a bunch of aggravation later, it might be worth considering.
Best regards,
Fred