"As far as 80cf. I thought that was the largest alum tank.And they only weight 37lbs filled."
No they make 100cf tanks but 80's are by far the most common. I haven't checked prices in a few years but you may be able to buy a few used 80's and get them hydro'd fairly cheap.
Dive tanks have burst disc's. If you nearly soiled your pants when a 1.5k burst disc blew out you'll stroke out when it happens and 80 cubic feet or all of the air in a small walk in closet decide to blow one out. Takes about 6 minutes to vent and sounds like a fright train going by