I've never used the fatboy zx6 shock, but I've also heard they're pretty nice. Get a shock pump for a mountain bike and play with the air pressure (preload) if you get one. Also be prepared to do a rebuild on a shock that old.
There's 5 preload settings. You'll likely find that you like a higher setting than I do. When i have a passenger, I like to ride with the 4th setting. If I ride solo with it on 4, it rides like a dump truck
A higher tail will make it turn in quicker, which is nice. with the new shock, it's only a little higher, not a lot. The change won't be drastic until you raise the tail a couple inches with shorter dogbones like I have.