I don’t this can be done without modifications. Anyone know for sure?

Nope. Not compatible.

Thanks Aidan.

The nuts/threads are different, and the tenon doesn't insert to the full depth, but the rate of taper is the same so you can friction fit it and try the combo quite easily... which doesn't quite work... cool sound, but stuffy.

The .500 bore slide on the 2B+ is really about as far as you can open things up with that TS etc. .508 is too much; the system balance isn't there and you crash into the resistance from the back end.

I have a factory custom made for Alan Kaplan that really is the "King 16M" - Jiggs bell, 3B slide - but it's properly different in between (different lower TS leg, the choke points at the start of the neckpipe and TS crook balled out, etc)... that does work.

If you can access a King 2B Plus, the 3B slide will fit. I've done this and didn't prefer it to the regular king 2B plus which is my fav of the 2Bs. You can also put the 2B plus slide on the 3B bell, which gives you a horn much like the Benge 170.