Man, can i recommend you trying tracklocross? I have put 40mm tires into a track frame with some angle grinding the outermost sides of threads on Schwalbe CX Comp tires and have 1 brake for some stupid off-road hills and to generally ride faster in the city to reach that off-road spots. I don't ride MTB, my other ride is road-touring, but having fixed gear training machine for a work week and that road machine for weekends struck a perfect balance for me