Hi Peter
This issue of more highway speed has been touched on before here is a link to some of them
https://www.google.com/search?q=%22H...i&cr=&safe=off
Now on to some of the points that I have experienced in 33 years of driving a 101wb HUP and some 20 years of driving 134wb C60S.
The 20" wheels with 1100x20 tire sould definitely get you up to highway speeds of 60 MPH on the flat with the 216 engine. But I'm not sure the 216 will be happy with sustained driving at that speed for 15,000 Kilometers or nearly 10,000 miles. Remember that your distance target is over the rebuild distance for the engine, though your miles should be cleaner and mostly highway.
Reason I say the C8AX will do 60 MPH is that my C60S with its original 216 engine would pull down the interstate at indicated 50 MPH and corrected road speed of about 60 MPH (rubber overdrive) suspect that if you run military tires you will be replacing them about halfway through your trip.
Once drove my C60S leading a convoy with my foot planted on the floor, never shifting, for an entire morning top speed 65 MPH running away from the JEEPs on the down grades and down to 35 MPH on the hills. On other long convoys the CMP didn't seem to mind it as much as the CCKWs. You could always touch the transmission and transfer case of the CMP with your hand and not get burned, you could not say the same for the CCKWs.
When I up engined the C60S to a 261 it will pull away from most convoys if I want has more speed than I can use. Which brings us to another speed issue with CMPs particularly the 101 and 134 wheelbase they get very lively at speed, think you will find that 60MPH is about tops.
Next noise and vibration, good noise canceling headset with an intercom to talk with anybody riding with you will be essential. Drive train vibration will be something that you will discover for your particular vehicle there will be speeds that everything vibrates and speeds that things seem happier.
I did drive my HUP for an entire year with both of the front inner axles removed. Which did improve fuel economy and reduced the noise. With the axles out the front end is not turning the differential or front drive shaft.
If you go this route I would have your rear axle shafts magnafluxed to look for a problem right at the end of the spines.
One more suggestion get a infrared thermometer and make yourself a temp chart for all the critical points. Then check the temps at the end of the days run. May really help spot a problem in the making before it becomes a trip stopping break down.