The weakness of the U1300L's is the portals. The diffs are strong, the gearbox is cranky, but strong. Finding a Mog with 23:9 "fast" axles, replacing the portals with the new AV portals and going to 365/80 R20 tyres (the bigger the tyre the more stress on the portals) will give you sufficient speed at a reasonable rev range plus you get the benefit of the stronger portal bearings. Adding the "super-fast" C&P will give you further rev reduction which may be a nice thing, but it won't make the truck any stronger, the portals will.
I just bought a U1300L with "fast" axles. What are those "new AV portals" you are talking about?