You have the skills. Just consider this a learning experience, and soon you will be building your own.
Plug and redrill. Don't try to drill on assembly; just measure and get the hole dead center on the bottom.
extend the backhead further aft with a brass spacer. Many steam locomotives had the backhead almost even with the rear cab bulkhead. You won't have to go that far - an eighth of an inch of brass, and some putty . . .
