Hey Nick. Welcome to classicbroncos.
With the right parts, you should have no trouble putting your old D20 back up behind a 435. Pretty sure that would be the easiest.
Obviously you could use a donor NP case (203 or 205) from behind an NP435, or even go whole hog and get an Atlas. Or even get something else.
But depending on what you intend to do I still think the Dana 20 is the best choice.
Do you have the intermediate housing (adapter housing) behind the new transmission? Do you still have the one that was between the old auto and the t-case? If so it should be pretty straightforward swap.
If you don't have any of those parts, we have this full kit:
https://www.wildhorses4x4.com/product/NP435_T18_to_dana20_Adapter/Bronco_3-Speed_Transmission that should help.
If you've got what you need though, you're in business.
I'm sure plenty of members here have done what you're doing. So someone will likely post up just exactly what else you might need. If anything.
Good luck.
Paul