Mike - I used the standard mounts. Although I didn't have a propane locker to deal with, it is difficult to get to a well supported point in that area. I would have liked to mounted the arch closer to the upper corner where the deck, stern and hull connections all meet but couldn't get everything to line up well there.
My kit came with backing plates that were perhaps 1/8' thick. These seemed a little inadequate to me - so I also created backing out of G10 connected to the hull with Evercoat Kitty Hair resin. This provided a flat/firm area that the then supplied plates were mounted. On the exterior, the upper mounts overlap the raised deck joint so I put a starboard spacer beneath the lip.
In mounting the arch, I created a bridal and used the main halyard to pull it up (I remember wishing I had left/kept some of the packing material around the arch legs to protect the boat on the initial fitment). I recall needing to use a couple ratchet straps to pull the legs into place and create the 'pre-load' recommended by Atlantic. I initially had doubts it would fit but it does and the pre-bend is important per the (limited) instructions. I'd recommend having 1 or 2 helpers - preferably engineer types and really take your time.
It was a lot of mental, physical and stressful work (as they say measure twice cut/drill once). You don't realize how large an arch is until you start swinging one around.
Good luck.