I'll put a couple of photos of my mounting method here, in case you can use any of it. Essentially, I wanted to mount it in a small, efficient space (in my case over the fuel tank), but as designed, you need a rather large open space above it to get it into its bracket and/or to get the cover off for (infrequent) adding of fluid. So I mulled it over and this is how I went about it. I should write it up in more detail in my "Sunbeam ~ 22 Cruiser" thread, but since that might not happen right away (ahem, I did this project two years ago!), I'll put some of it here now in case it is helpful in any way for your install -- you might have the same situation with "low headroom."
To put it in a nutshell, I made a (yellow) Starboard™ backing plate, and mounted Bennett's bracket to that, "permanently." Then I used Weld Mount studs to mount the backing plate to the hull. Now I can just pull the backing plate on and off the studs to service the unit and don't need to lift it up (into clear space that I don't have in that location). There is a bit more detail in the photo captions in my album (sub-album for trim tabs).
As you can see, the fuel tank is out (I had a few additional irons in the fire), but I finished up the installation with the fuel tank in place so I could be sure it would all work out once things were put back together. Just to give perspective, the "roof" above the HPU is the underside of the starboard lazarette.
This was all done a couple of years ago, and so far has been working out fine.
