If his cabinet is real old like 70s or back he'd be nuts to take it apart as its worth money.
As for the cabinet I'd go by the same exact dimensions,wood type as a Marshall cab.Also there straight cabs aren't straight the face that the speakers mount to is angled in the cab.
Marshall used Celestion speakers(I prefer the 25watters cause there smoother)Wire the two left speakers in series positive from the jack to positive on the 1st speaker out the negative into the positoive of the 2nd speaker then out the negative to the negative poll on the jack.Then just do the same for the other side.Wired like this 4 16ohm speakers=16ohm cab.
As far as the stereo-mono switch unless hes splitting his cab with two heads just stick the single jack on and be done.