Okay, so I took some shots of the 40" side of my tank, so just a little longer than yours. My tank is also 18" tall, so at 3" deeper I'm sure your sandbed will have pretty decent coverage.
I also kept in mind that with the height of the fixture itself, and leaving some wiggle room for the cord, it takes up about 2½" of space.
So this is the light mounted at 3". With the height of the fixture, the top of it is 5½" above the tank rim. Distance from LED's to sandbed is 21". Noticeable cone shape, hard to see the return lines on the left, rocks on left have poor coverage.
Light at 6", 8" inches from top of fixture to tank rim, 24" to sandbed. Koralia clearly visible, returns visible, coverage isn't too bad.
Light at 8" above rim, 10½" from rim to top of fixture, 24" to sandbed.
I think this is good coverage, and it's where I usually have my light mounted anyways. The photograph shows fairly even spread with not a huge amount of contrast, and this is much less noticeable to the naked eye also. If you can swing this height, in a 36" wide tank, I think you'll be happy with the results!
Anyways, I think you're in good shape. Depending on the rockwork, a 36" tank might be acceptable at the 6" height, I guess depending on how picky you are, but the light is badass anyways so you're sure to enjoy it!
Looking forward to seeing the tank filled and lit up!