Rooney I can't measure currently but my '86 is running 35mm Paselas with 45mm fenders and clearance is fine. Definitely maxed out, but fine. Could go up to 40 without fenders I'm sure. I believe I do have more clearance in the fork than the stays, so your theory could be correct.
This article was already posted here I think but this guy has 45mm up front and 42mm in the back on his 86, although the back is far too close for my comfort haha. No fenders of course:
https://bikepacking.com/news/readers...winn-voyageur/