A clement xplor MSO 700 x 40c fits and spins, but theres only a few mm of clearance either side at the chain stay. A buckled rim might be a ride-ender.
Also the chain stay and seat stay bridges are also too close for comfort (no chance of guards and very little mud clearance).
Realistically a 35c in the mud is all you would want to run. You would probably get a mudguard in for road use with a 35.
A shame really as there is bags of width at the seat stays and it would be dead easy to DIY crimp the chain stays for more room there.
Just the bridges are too close for comfort for a 700 x 40c. Still its a bargain frame for commute/cx/adventure, just maybe not monster cross!
Having said that, if you run a 650B wheelset, wearing 650b x 40 gives a similar rolling radius to 700×28 and the frame would have enough clearance with the tyre in that position (5mm either side?). A 650b x 50 (650 2″ mtb) would leave plenty clearance at the bridges, but you would want to DIY crimp the chain stays to get 5mm clearance: https://www.youtube.com/watch?v=oNX90sKhg0Q