Eaton 13spd aux box install

Some you can take rear bearing caps off, make a plate and bolt counter shafts to plate, and housing.

 

It can be a bit tricky getting it together with the transmission installed but its doable.
Remove both the big dowels and clean the holes on both housings. Glue the gasket to the rear housing. Remove the rear bearing covers from the rear housing. Make sure the range fork is all the way back, engaged in low range and the splitter is in gear.
I have always used 2 long 3/4" bolts in the dowel holes to install the rear section.
Lift the rear section up into place with a transmission jack and get both 3/4" guide bolts in place. slide the rear section up to about 2 1/2" gap between the main and auxiliary section and remove the jack. Get underneath and remove the right guide bolt and lift the auxiliary section with your knees so it goes in about 1/2" high on the right side to engage the gear teeth.
It should slide into place. Put the right guide pin back in and put a few bolts in and snug them up not tight. If it wont slide in try lifting it a bit higher. The counter-shafts droop a bit from bearing clearance and how loose they are in the housing.
You should be able to gently tap the rear counter-shafts all the way ahead in the rear housing. The yoke should rotate freely with no binding.
If so put the original dowel pins in and all the bolts and tighten them. You should be good to go.