would 26"x1.8 be high enough? - ie is there a risk that the BB will be too low? Especially as touring frames tend to have lower BBs.
The chainstays may be worked around by getting a frame builder to put dents(?) in for tyre clearance. The seat stays may be more of a problem (assuming they are a problem, which re-reading your post they probably arent)
would 26"x1.8 be high enough? - ie is there a risk that the BB will be too low? Especially as touring frames tend to have lower BBs.
The chainstays may be worked around by getting a frame builder to put dents(?) in for tyre clearance. The seat stays may be more of a problem (assuming they are a problem, which re-reading your post they probably arent)