I need to purchase a rear buis for an upper with a non F-marked front sight base.  Are there some that will work and some that won't, or do I have to buy a taller front sight pin.  Will the YHM-9680 rear buis work?

Pretty much all rear BUIS are the same height.  I don't know of anyone who offers different heights for the F and non-F FSB, so you'll have to use the different height front sight if it doesn't work.  The YHM should work fine - it's one of the best less expensive folding BUIS.

Note that there are FSBs that are the F height but aren't marked F.  You might want to measure or shoot it before you change anything.

You will be able to zero with just about any rear.  The front may need to be adjusted a tad more than if it were "F" height, however there should be plenty of elevation on your front sight as is.  Worse case is to get a taller front sight post from Bushmaster if you find it needed to zero.

ARMS 40L works fine on my 6520 converted to flattop with the non-F marked FSB and stock front sight post.