[BB_TX]

I would go more than 40'. That 3400 is over 37' long. I would want at least 3' front and rear just to get around it easily. If the building is 40', subtract the wall thickness, and you would have very little clearance inside.
I have not built one (yet) for the Monty. But I did build a 30' x 40' metal building on some land we own to keep my tractor and other things. Bought a prefab kit and built it myself, with only a little help from some friends. Has water, electric, sewer, bathroom, small kitchen area, workshop area, fully insulated. Worked out great. But that was 10 years ago. Don't think I would do it again myself.
Mine only has 10' walls and about 15' at the peak. Would need much higher for a 14' high door to clear the trailer.
There are lots of companies to choose from. But I don't know much anything about any of them except who I bought the kit from. But they don't install them themselves.