Here is a website that has some Dash 9 proto specs that may be of interest to you folks. It doesn't have the rail to tank height but it does have other measurements that may be relative or helpful in your quest for a realistic looking height. I can't speak to their accuracy but thought I'd pass the link on just out of interest.
I do not own a dash 9 but fwiw when I have looked at them (the 1:29 size) they do seem to be a bit high on the rail. Beautiful loco tho.
Raymond, may I also add to others comments that your modifications look terrific.