Oh FFS do it right!
So let me get this right, a 19" rack is too small, so lets go non-standard for only 4" more?
Why not go for 2*19=38" wide and be done with it, so you can mound old and new stuff in one rack?
Somehow I doubt that a few inches are so make-or-brake for cooling, and the real issue is just what is in a box and how it is cables. Most racks seem to end up messy for cabling, more so if you have servers on sliders to gain access and so have big loops on supports arms at the back. Why not have some "plug in" rack so the inter-unit cabling can be fixed, and the unit pulls out completely for repair, etc?