Problem solved. I went to the local hot rod parts store. They serve so many flatheads around here that they carry the fat belts and were able to order them from their warehouse and have them delivered later this afternoon. Turns out a 43 inch belt is what I was sold as std for Model A, and I believe that is correct. A 42 inch belt gives the needed clearance from the hose in this situation. The powerhouse looks a bit upended with the cutout kinda tilted instead of sitting where one would expect it, but it fits. A 42.5 belt might look better, but I'm not so sure it would have adequate clearance.
Thanks to all for your advice. It helped.