I talked about this a lot with Yabanja who was chasing weight and balancing issues with his swb Fuso. He came to the conclusion that especially with a shorter wb that staying under 50% was really necessary for handling capabilities (and thus safety?). No intention here to quote/misquote him but that was my takeaway. In my case this wasn't a deal breaker as I have other size limitations (a shipping constraint from my employer) but I am struggling mightily to find a way to get two full size mounted spares somewhere other than on the back wall. With a rack of any kind these are over 350# together and that's a huge amount way out on the end of a Defender-size frame in my case.
Fwiw, if I and my rig were both in NA at the same time, I'd bite the bullet and extend my rails between the wheels in a heartbeat. Likewise, if the 14' ambo box can't or shouldn't be shortened based upon your design needs, I think an extension would be money well spent for yours as well.
Alternately, break out the scales, spreadsheets and math and see if there's some way you can accurately account for your weights and get probably 75-80% of your total box and add-on weight forward of or on that rear axle? It's a wicked problem, because of course the wheels being close together also limits the under rail storage for water, batteries, and even fuel. Things like chains, heavy tools, even spares maybe could be considered for location in/on or in front of the cab as well to make a big change in balance. The guys in OZ will tell you that there is almost never enough weight over the front wheels in these cabovers for the front wheel traction to be comparable to a normal 4x4 or ute, especially on any kind of slope or climb.
