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.