It's difficult to find a modern stock factory frame with wide tire clearance and a quill stem. SOMA is an anomaly here. If you have a broad budget you could look at the Rodriguez Shiftless @1k+ or the new ssfg coming from Rivendell in the next few months @1.2k. The shiftless will need to be customized but that won't be an issue.
The most simple solution is probably an 80's road conversion, like a trek 460, for example which would be pretty solid off-road and would easily clear any 622mm cross tire. Many early 80's road bikes were designed around crit racing and feature short chain stays and 74+ parallel angles, but with lower than track bottom bracket heights, 85 iron man centurion, for example. Both of these have quality steel (Reynolds 531, tange 1) and are lugged...for ego points and should come in well below what you would pay for a rush frame.
If you're planning to spend more time off road, you could consider converting a mtb of similar vintage. You'd gain about 1-2 lbs in frame/fork weight but you'd gain canti brakes and the ability to mount just about any wheel you like.