Just a guess but if the tongue is 1000 empty those axles are going to be pretty close to centered to just slightly rear of center.
I think a good spot for the vet may be on the bottom backed in. Bikes & quad up top with the quad forward. That would also offset any potential items stored in cabinets or up front. Initially one just has to experiment and see. What fun!
You will have the luxury of always hauling the same stuff so with enough effort you will be able to test loaded positions and come to the most convenient way to overcome the tongue weight issues and just concern yourself with the gross weight. A vette a couple of HD's and a quad is around 7000lb net wt.? That is not real outrageous. The trlr. itself is another 3 or 4000?
A morning at the scales will answer all.
Where is Toy Box or Kevin and Jack they will have some good advice.
To the hitch question I would get the best I could find. The equalizer hitch may help some I suppose. Wouldn't hurt.
I recall someone posted a trailer tongue weight scale a while back, Kev I think. It went under the base of the leg.
Ditto what Harry said, the trans will be the weak link. 65 or 70,000lbs is a lot for an automatic.
What is the most gross weight Allison suggests safely pulling with one? Is there any additional aftermarket coolers available from Allison????????????? With the way the external filter is plumbed a guy could probably fashion something pretty helpful for dissipating heat by tapping into those lines. An additional external cooler with d/c fans with thermal sensors.