You can purchase the base only for the new 20,000/5,000 lb rated RVK3500 Companion and use your existing hitch head from your 18,000 lb rated Companion 5th wheel hitch. That's what I did when we upgraded to our current 19,000 lb GVWR Mobile Suites. The instructions for assembling the new RVK3500 along with the different adjustments and resulting dimensions are shown
HERE - dimensions on page 2. I don't know if this will get you what you need, but you might want to check it out as it's MUCH less expensive than buying the complete RVK3500.
Unless my memory fails me, the part number for the base kit was RVB3500.
Rusty