I've been considering a similar rifle for a while now. Can't see it being overly practical for me, but worth trying anyway.
The only mags that would work are AR mags, which means they're pinned to 5 anyway, unless you can find an AR pistol mag in 6.8(not sure if they're available here).
Mossberg is set to release a rifle that takes AR mags, but only in 223 for now (just found it on the firearms blog), so there is hope. At least this is a better idea than a 30-30 with quad rails.
I don't know if you're set on a 6.8SPC, but I've been leaning in the 300Blackout direction, just for mag compatability, using the available 10rd mags. Just need to find some bottom metal.
I think it was Kevin M on here that did his own conversion of a Rem 700 with a 30rd mag. You might find it in a search.