Another thing to really consider here is ventilation. While I do not know all the intricacies of DIY tanks into walls, I do know the damage that can arise from poor ventilation. 300 gallons of saltwater will produce a good bit of evaporation (although it could be much worse, just ask Mr. 4000).
Just throwing out a detail that many forget with these type of setups. I wish you the best of luck with it, I can't wait to see its progression (BTW, I hope you have a digi camera for all our curious eyes
).
Take er easy
Scott T.