Just as I had done the calcs as well. However, the amount of cobble would actually be 79626240. (64*45*27*1024)..
It is more, because barrels are limited by number of items, not number of stacks. So it actually would be 64 in stack * 45 in backpack * 27 in bag * 65536 in barrell = 5,1 * 10^9. If you need more storage you can always configure adventure's backpack to accept RP2 bags.