To my knowledge the Jag is the deepest stem available as a quill at 58*, but only comes 25.4. I'm not even sure if threadless stems are available past 15*, however irrelevant that is.
If you need a 26.0, you may be stuck with a Pearl. You could always go Look ergostem, but that would be horrendous on a vintage bianchi build and really expensive, so I apologize for even suggesting it.
I'm sure you'd be in a similar situation regarding price for some Mod 67 cinelli drops etc. I'm not even sure what the clamp is on those anyway.
In conclusion, I have no constructive suggestion to offer.
The frame (which is not yet in my possession) came with a 3T road stem and Cinelli 67-39 bars. I find this strange, since I don't believe 3T ever made a stem with a 26.4 clamp and I'm not sure if the 67-39 was ever made with a 26.0 clamp.
Ideally, I'd like to find either a 3T Pista or Cinelli 2A (or 3A, if it really exists) stem, but I haven't been able to find one in the correct size. I was hoping there was an alternative to using either an Italian road stem or a Nitto track stem, but it's not looking good.