My experience with my 650B Hilsen and Silver sidepulls.
- Hetres with no fenders is spectacular. It looks awesome.
- Soma New Express with P45 fenders is great
- Hetres with P45 is super snug but I used it with no rubbing. The two tightest spots are the two you have identified. 1. headroom under the fork crown 2. headroom under the rear brake bridge
- On a whim wanting to make my Hilsen more stylish I switched to Honjos. The choices I made on how to attach them made the two tight spots even tighter. So, as I have them set up now, I cannot run Hetres. I have Pari Motos and a stock of Lierres for the Hilsen now.
- As a winter project I'm going to return to SKS fenders on the Hilsen. I am content that Lierres + fenders will be a splendid set up for the Hilsen for the long haul. Jan Heine himself assures us all that the Lierre is just as good as the Hetre (at least really close). If you have to have the red-tread, the Soma B-Line comes in red and is a great tire, in my opinion.
In short, my 650B A Homer Hilsen is the finest bike I've ever owned, but I'm now convinced that the clearances it was designed to provide are spot on PERFECT for 38s with fenders. 42s with fenders is a tight squeeze. I have 2 other bikes that I can run Hetres on. If I was dead-set on running Hetres on my Hilsen, I'd either resign myself to it being really tight, or I'd think about getting it converted to a canti-Homer and get a repaint.