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That route is good, I have done a very similar ride (Abergavenny to Machynlleth) over 2 days on "gravel" and small/unpaved roads last year on my touring bike. It was exceptionally good fun and surprisingly tough, not helped by us doing it foul February weather, here are some pictures of the pass above Hay on Wye, which has the most amazing descent into the town.
I have a 2.3" high roller on hope tech xc wheels on there currently. wouldn't go bigger unless you were sure of a no mud ride. sorry don't have calipers to measure it up or anything. tbh chunky 2.3" is enough for almost any riding that's no fatbike 4" territory.
I may well pinch that route thanks. I want to try this too:
http://www.bikepacking.com/routes/trans-cambrian-way/
Last question about the Cotic, what size tyres can it clear do you think?