As I stated earlier, one thing that helped was the availability of cab components from scrapped E7's - I assume throttle, brake, electrical, etc., components came from those or similar units, along with the door assemblies.
But consider what had to be installed to make a working cab: pilot, floor at the proper height (engine room floor is not), bulkhead and engine room doors, electrical cabinet, FRA-compliant windshields and collision posts (ca. 1973, anyway), aforementioned controls plus piping and wiring.
Even though it was dead air space at the B-end of a B unit, it was probably still cheaper to torch the carbody back to where the bulkhead and salvaged electrical panel could be installed and start over - especially because the B end was likely not collision-compliant (I'm inferring here, as I don't know the standards of the day). Plus, fabricating straight pieces of sheet metal would have been fairly fast and inexpensive.
The main thing, though, is that it wasn't possible to cut four windows in the B end and plop a control stand in - too much additional equipment to be added.
Clear as mud?