I don't think the bearing blocks have to slide quite that much. Adding 2 chains of even #35 chain only adds .375" to the C-C distance, so the bearing blocks have to move no more than .375", unless you're using chain bigger than #35. Even adding a bit more for safety, that slot looks larger than an inch!
I don't think the WCP bearing block cam can even go that far.
All these huge slots might end up compromising the strength of the tube where they meet. The bearing block itself might help reinforce it a bit... probably not. The tube could bend or twist where the slots only leave 4 thin corners. If the huge top slots are only for poking at the chain/belts in the tube, they can be smaller.