If you are considering a 6th, I would think mixing the HT & HST would not be a viable option. 3 chambered ported is far removed from 6th.
The ghetto blaster Suburban build had 6 HT 18's in a 4th, if you'd like to see other SI builds.
I'm not an expert, but I'd def ask those that are before you invest tons of time.
Where the Governor is on a steak, the cops are on the take, and the welcoming committee is a sawed-off shotgun!