I would like to rebuild my 6921 clone with perm attached vortex. It currently wears a kac rail that I would like to not destroy. What is the best way to about putting a 13 inch Geiselle rail on without messing with the FH? I am thinking about cutting down the fsb, or cutting it off if it is possible to add another block without removing the FH. Thoughts/ideas on the easiest/cheapest way to do this?

Nobody?

Grind down front sight (harder to do without taking it off) or install a two-piece gas block (another).

2 piece gas block is your only choice and you'll have to cut off the stock FSB which is going to be a bitch.  Can't get a low-pro gas block on over the muzzle device and can't get the new barrel nut on without removing the stock FSB.

In all honesty, it'll be A LOT easier to just remove the pinned FH, swap your rail stuff, then have a new FH pinned back on.