I know of at least one boat which had a custom or semi custom built tank which is aluminum (I believe) and is larger than the two tanks which are the standard.
A number of boats put the drain plug in from the outside--so that is not an issue--the bilge pump can be, but there are farily simple ways around that--one of which is to use a diaphragm pump, with a pickup tube in the lowest part of the bilge aft. (Jabsco made/makes such bilge pumps, and they are true displacement pumps, rather than vane type of the cheaper Rule pumps.)