Most truck chassis have the ability to mount several steering gear boxes to the same mounting holes... SAE (society of automotive engineers) in the HD truck industry has worked effectively to allow chassis builders several options in types of axles, drive systems that can be mounted in a chassis... there are at least 20 very good HD rebuilders in the industry across the country... they will have a cross charge to switch from one brand of box to another and in many applications can give you a HD unit that is an upgrade from the one you have...
When doing any kind of work like this ... always, that's always install a remote filter and spin on cartridge to protect both the pump and gear box... this is very cheep insurance for something the factory should have done... you not only filter the oil but usually add over a qt of capacity that allows the unit to run cooler...
Do a search for HD truck steering gear box rebuilders... scope out their web pages.. BTW most truck dealers, RV dealers, and auto parts stores have no clue... these rebuilders usually service independent repair shops and large fleets...
An example of a company I've used and trust... BAB Steering Hydraulics
14554 Whittram Fontana, CA 92335.... you can google them and see their web site..
Hope this helps.
Retired Business Owner, Re-manufacturing HD Clutches, Brake Shoes, Air Compressors, Sales & Installation of PacBrake and other Industrial Friction