I just bought a 48 gal long that sits 17" tall. I am working on slowly moving my BP into that as well. To keep up your temps try the following
1) make the lid out of plexglass, and cut holes for you heat lamps to fit in preventing as little heat escape as possible, make sure you put wire mesh under the lamps to prevent burns
2) A UTH should keep your temperature high enough, make sure you are taking your temperatures inside the dens, as the temperature inside a den on top of a UTH can be more up to 10 degrees hotter than outside.
3) I have 2 red lights heating a 48 gal. long right now, one 75 and one 100 I think, and they both work failry well to maintin daytime temperatures at 81 and 85 outside the hides give or take 3 degrees, and inside the hide on the hot side 93 give or take 3 degrees.
4) Another trick is if you 55 Gal. is not a long tank (in other words failry tall + 17") when you do work to it, give home 3-4 dens, place two dens, part way up the tank on say a second floor, this will put him clooser to the heat lamps.
Also, you mentioned a water fall, ive been trygin to thikn of different ways to do this, and I was wonderig hwat your plan is?