As G.M. said, the later model Columbia's are very easy to adapt into the '35-40 Fords..
My '36 had a '36 Columbia in it originally, I converted it to a '42-48 unit using a combination of stock Ford and Columbia parts.. Using the right combination of parts eliminates any cutting, welding, etc.. The '42-48 Ford/Merc Columbia's are only 2" wider than the earlier units, an added inch on each side is not a big deal..
I used my stock center section/drive-line, '37 radius bars with 3/4" thick nuts as spacers where the bars connect to the torque tube.. A 42-48 Columbia right axle housing, stock Ford left axle housing, stock '42-48 axles, '41 spring, '48 brakes w/39 drums.. I even used the stock sway bar mounted to the right backing plate and rear cross member..
I used the '42-48 electric/vacuum switch, cutting the head mounting bracket down so it would mount out of sight on the fender splash panel.. Mounted the clutch safety switch on the engine head using a portion of the original bracket.. Mounted the speedo switch up under the dash on the pass side of the firewall, directly inline with the speedometer..
The Columbia switch is located in the lower edge of the dash, out of sight..