You can run a butt load of 1708 in there but why? When sealed correctly it wont get wet so there is no worry there. And plus all that fiberglass you will need to get the same strength will far outweigh balsa core.
Long story short, Just use balsa core. Its lighter, stronger, and way faster to put down. Just seal it up good and there will be no worries of it rotting out.
Here is an ok picture of my Vulture with the core down the pad.
