instead of adapting the TH400 to the D300, why not try and find a TF999/727 from an 80's CJ that would already be adapted to the D300? Do you already have the 300? if not you could probably just buy the tranny/tcase together in a set cheaper than you could find a D300 and buy the adapter.
1985 CJ7 Project Phase One : 5.3L Vortec/4L60E, Moser axles for AMC20, Twin Stick'd Dana300 - Completed Phase Two: Bedliner, Rollcage, Seats, BESRK Rear Bumper w/ tire swing, Soft Top, and a few other goodies - Mostly Complete Phase Three: Waggy 44 front, Scout D44 Rear, 4.56's, ARB's (F/R) SOA with YJ Springs, 35x12.50x15 BFG MT - Not Even Close to beginning
Also a mostly rust-free 1970 Scout 800A That I'll get to start working on someday...
Chrome won't get you home...BUT Rattle Can Black may just get you back"
I just did the swap and it really inst bad at all.
As the guys said you will need a centered rear axle (which you have)
The TH400 used in front of the Q Trac has a 10 spline output shaft that sticks out about a foot from the back of the trans so nothing bolts up to it. The bummer of the swap if you could call it that is that the TH400 has to be completely gutted to swap tail shafts and is a good time to throw clutches and seals into it.
With the novak adapter my trans mount holes in my q trac cross member bolted right up.
You will then need to get front and rear driveshaft’s made. I have a spare Curtis Tatton non CV style rear with 1310 spicers in it that should fit if you do the swap that I would sell real cheap. I only had about 25 miles on it.
I have a lot of this documented in my build thread as well.