Yes, the 700R4 factory adapter will work as it has the same bolt pattern as a TH350. The bolt pattern on the TC side is, as you state, identical to the D300 except for 1 bolt. I've seen multiple suggestions as what to do with that misaligned hole. Some slot the hole out and use the stud on the D300, some have redrilled the TC, and some have removed the stud/bolt entirely. I slotted out the hole and used that last stud just for peace of mind. I would suspect removing that stud and not using that hole would be just fine. Some 350s have different output shaft lengths. Be sure that you measure your setup as you'll need to call Advance Adapters for a spud shaft to mated to the TH350 and the D300. They have different lengths depending on your output shaft length and if you use a clocking ring or not. Be sure you have sufficient spline engagement so you don't destroy the shafts. I think rule of thumb is spline engagement should be AT LEAST as much as the diameter of the shaft. I'm sure there is a much more technical way to determine this but shoot for at least that or more.
The spud shaft is short, so its likely that you won't need a carrier bearing. I didn't use one on mine. Make sure your seals are good so the fluids don't mix. I packed my adapter with gasket packing to support the shaft a little and to help seal it up to keep the fluids seperate.
