I installed the Fusion AV650 head and replaced the speakers with 6/4" Fusion as well. Far superior. Fished the cable for the remote unit under the headliner as the stock radio and installed new remote unit in same spot. Came out super.
Looking at the 4 channel amp, 3rd set of speakers and subwoofer. Likely will put the speakers where they do today on the new 300 (walk step area) and relocate the cockpit lights next to them and the sub aft to the starboard side of the aft seat.
The Fusion head unit and speakers were plug and play with the existing speaker and head unit holes. Got a plastic plate to make the install look perfect. Did have to open up the mount area for the remote unit, but went right in the same place. Looks like it came from the factory after the install.
If you keep the Kenwood. Inspect the units good, they rust up and there isn't conformal coating on the circuit boards. Check ebay for a new remote if needed. I got one for $10 a few years back. Fusion is the way to go if going to splurge.