Shameless plug.
PRF Celt 912
PRF Grid Master 723
Flushing my 323 cow to 723. Using 912 is working and 1st calves due any day.
Aside from my own, I really believe anything out of the Grid Maker lines would work well on 323's. I really like Grid Topper. 323's seem work well with a bull of bone and guts.
6807's ought to be bred to EXT lines, that cross has been around for a while and works well.
Sizmic