Our tires, GOODYEAR G178 - 425/65R22.5 have a speed rating of 68mph. That is plenty fast for us. I don't expect we'll run more than 65 very often. I am reviewing the manufacturer's Load Adjustments for Speed worksheets and data. It looks like you've spent some time studying this kind of data.
FYI, the following represents our current GVW calculations:
14,333 Truck Chassis
5,500 GXV Cabin
24,233 Total Est. GVW (73% of GVWR)
Regarding the width of the rear axle, we spent quite a bit of time working with the engineering department at International to develop the spec'd rear axle. It is a single axle MERITOR MS-21-14X-6DCR Single Reduction, Wide Track, 21,000-lb capacity "wide-track" axle. They have assured me that it will have the identical track as the front axle.
I hope that you have a great adventure down under. Ron (GXVMogHvn) has just returned and it sounds like he had an incredible time.
Thank you, again, for sharing your wisdom with a novice like me. I am in your debt!