What kinda bullhorns did you get? Most of them are 26mm clamp diameter and your stem is 25.4 clamp diameter so you should probably get a 26mm clamp quill stem also. ALTHOUGH there are a couple of 25.4 bullhorns out there.

What kinda ride you got, that lug actually doesn't look to shabby.

Oh, wait you said you already got a new stem? I'm assuming threadless. What stem did you get? There are adapters that you can insert into your steerer tube and clamp your new stem to.

You'll either need a 1" to 1" threadless adapter or a 1" to 1 1/8" threadless adapter depending on your stem.