I think the 40b is an awesome tank size. Just get a tall stand to go along with it. My 40b is right next to my computer and is the perfect height.
The 50b would be nice as well despite being very hard to find. I would say no to the 65 however as it's hard to light a tank that tall.
, a 36" tank just isn't right for african cichlids. They really should have a 4ft long tank.