Trouble with Temperture in a 150 gallon enclosure

I recently "rescued" a big male boa who was going to be euthanized after he was seized by the police and the county was unable to provide him with care (every cop in the station was terrified of the poor boy, don't know how they even got him in a pillow case).
Anyway, my friend in the police force threw my name out there and after a few papers signed, he was mine. He's a big, beautiful male (not sure of morph, I'm awful at that kind of thing) and he seems to have a slight upper respiratory infection. He opens his mouth occasionally, like a mini-yawn where he only cracks it open a few centimeters for a second or two and spends a lot of time basking.
I'm guessing his previous owners didn't care for him very well, but the inside of his mouth isn't "cheesy" or mucus covered or anything so that's good. In the past with ball pythons I've used Baytril to clear up URI, and if the heat doesn't help it in a few days I'll try to procure some.
He seems to "hiss" a lot, even when I'm not in the room and I can hear it very clearly. It almost sounds like snoring, with every breath but it only seems to occur when he's basking. Could this be actual hissing or a side effect of the URI?
The 150 gallon tank is glass or acrylic I'm not sure which, probably acrylic, and it has a screen top I've weighed down with cat litter and unused aquarium sand. I have a 150 watt and a 100 watt Zoomed basking bulb shining directly on the log I set up as a basking spot and he spends almost all of his time directly underneath those lamps. I do have extra fixtures, so I can install another bulb if needed I just don't have any more bulbs on hand. The under tank heater I used to use on this setup is busted, guess it didn't store so well.

Any tips on how I can warm my new friend up? He's alert and relatively active (he has, after all only been in the setup for a little over two days now so he's probably still exploring). I'm worried about the URI and I'm tight for cash so my usual vet might have to wait a bit if there's something really wrong.

Anyway, heating is my main concern. I'll try to setup a thermometer and get some temp's but for now it just doesn't seem to be heating up enough for him.

I would go with a UTH + thermostat before adding another light source.

A 150 gallon tank is pretty tall. Can you put something sturdy on the floor of the tank under the heat lamps that he can climb on so he's a few inches closer to them?

Also he'll need higher humidity if he's got a URI, I would put several water bowls in there and cover as much of the screen top as possible.

Yeah that bad boy is 25 inches tall, it's a nightmare getting lamps to cut all the way down. I got a very big log about 8 inches under the basking spot to allow him to get up close and warm up.
I'll try and pickup an under tank again soon.
I've got a large plastic tub, about 18 iniches long and 12 inches wide, about 8 inches filled with water on one side (cool side tho) and I'm gonna try fixing my old power mister so that I can help with humidity.
Thanks for the tips, I'll see what I can do for him.

Probably could have taken a better image, but here he is basking. This log is about 8 inches directly under his lamps, so I think his basking spot is good but ambient temperatures throughout the cage are a nightmare
http://i.imgur.com/dHzZJcf.jpg

How big is he? Honestly trying to get the right humidity and temp gradient in a 150 gallon tank is going to be a nightmare and require more cash and effort than it would be to just order something like an Animal Plastics T8 as they are a quick ship item. And you mentioned ball pythons so later once you get him all better and if he needs a larger cage, just order a larger one and give the T8 to one of the BPs. Heck you could probably sell the tank once you get him in an AP cage and recoup some of the Ap cage costs lol.

Looks like you got the basking spot good. I would do like bcr said and add a UTH. Get one that covers 3/4 the cage floor and set it with a T-stat to around 80-82. That way you can cover 3/4 or more of the top with thick cardboard wrapped in tin foil or if you plan on keeping the tank, measure the top and have a glass or plastics shop cut you a piece of acrylic and drill some holes in it for ventilation.

And as for the sound he makes, it sounds like the RI is a bit worse. I would definitely get him to a good reptile vet. Baytril is pretty harsh stuff. I would get a culture done so you know exactly what you are dealing with so you get the best antibiotic for him. Also wont hurt to just get a checkup. I usually take my gals and guys in every year or so for a checkup.

Anyways good luck with him. At least he is mellow while being sick which is a plus lol.

It would look awful but you could get some rigid foam insulation board from somewhere like Lowes or Home Depot, cut the panels to fit the outside dimensions of the tank, and attach it to the outside. A 4x8 sheet of it is around $15 so it's not expensive.Otherwise your only easy option for increasing the ambient temp is to increase the temperature in the room.