There's nothing currently installed. It was only after taking off the Middleburn square taper 32T cranks and removing the BB that I suddenly thought "hmm, I wonder how wide that BB shell is?" and that's when it dawned on me that the cranks were for 68mm and the shell measured 73mm.
Both 801 and 501 will fit, 801 will need the plastic TL FC25 tool which will fit any regular HTII BB tool, basically a step down washer.
Surly says Chainring Clearance 24/36/48t so you should be able to clear quite a bit more than 36T.
Nothing beats actually trying one on though.
What's currently on there?
Photo of full biek?