The oil pipes are in the way of a box wrench from the bottom. With the clutch housing removed a box wrench could be used.
The seal housing flange is 2.5" diameter (the snout that actually holds the seal, not the part that the screws go through).
Distance from center of an 18 tooth sprocket to the midpoint of the side plate is 1.772" so there is 0.522" from the midpoint to the seal holder. I think any chain will clear that.
The problem with hitting the seal holder usually is the width of the chain, some have very thick side plates.