i have a similar dual exhaust system on my FHC and yes, it is loud. The loudness is a matter of what muffler you have. My muffler is heavy stainless with no internal fiberglass stuffing for sound dampening. My pipes are aluminized steel. I am staying with this system for now but may in the future switch to a much quieter muffler that Bill Bassett supplies. I like the performance aspect of the virtually straight through exhaust versus the single system.
Regarding the mounts, the first mount is a bent L shaped piece of bar stock that mounts to a tab on the pipes located in front of the muffler and attaches to a tab on the frame. The second mount is an extended channel welded to the top of the muffler (shown clearly in you pictures) that attaches to the U shaped bracket via a rubber spool piece. The rear attachment is a sandwich of spacers that attach to a Z bracket that bolts to the underside of the spare tire tray. All the mounts are clearly shown in Viart’s XK-120 Explored book, if you have it.