if you're used to riding in the drops of drop bars you probably want an even deeper drop stem for bullhorns. If you're used to riding on the tops of your drop bars, you might want to stick with your current stem or get one with more rise. Theres tons of different bullhorns with drop and no drop so the stem you use will depend on your bar choice. I prefer a stem with a bit of rise, combined with bullhorns that drop, which means that I have a nice high hand position up near the stem, and a lower position when I'm holding onto the horns (kind of like drop bars in a way but backwards).

Your main problem will be that nitto stems and bars are 25.4mm and almost all bullhorns out there will be either 26.0 or 31.8.

There really is no answer as this is a fit issue and everyone is shaped differently.

You could just chop and flop your b123s. That would be cheap.