[Arran]

 Since it's an empty cabinet anyhow there is really no harm in doing as you suggest, now if the original chassis and speaker were still with it, or if it were a cabinet for a rare model I would probably say otherwise. However I would not totally abandon the idea of finding an original chassis and speaker for your 37-640's cabinet, so what I would suggest is making any modifications reversible such as by using the original  holes for the controls and chassis bolts, it will look better as a finished product anyhow. With the speaker it doesn't really matter since the baffle board is easily removed and replaced by another to accommodate whatever size and however many speakers you can fit behind that grille.
Regards
Arran