Hi There is there any reason why you arn't just looking at buying a preformed aquarium?, you can get them in large sizes and removes all the worries and calcualtions.
Hi I would definatly look at switching to an external filter, in my experience they just better for larger aquariums and not to mention easier to maintain.
I would think about doing the following:
Run the new external filter and internal filter alongside each other (turning down the outputs...