Thank you everyone for the thinking. Ironically, the jack handle hitting the tank wasn't a worry for me, because I can either a. life the handle up about 15 degrees and then it clears the tank, and b. If I use a 10lb tank that is shorter than the 20lb one in the pictures, then the jack handle will clear easily. Plus, as pointed out, the tank is currently sitting in a TankFoot and that's raising it up a couple of inches.
Here's what I'm really concerned about:
1. Is this too close to the tire and jack? I'm guessing if everything clears and I can remove the spare as needed -- no harm... no foul.
2. I'm going to have to go to a metal shop/welder to make/mount a bracket set that will securely hold the tank while in transport. What kind of mount should I ask for, and basically what do I ask him/her to do? I couldn't find the picture, but someone in the CTC forum built a square aluminum frame with metal rods that went through the base of the tank, securing the tank. Is that the way to go?
3. I like what Astrotrailer was saying about having brackets welded underneath the tongue and letting the tank site into the tongue between the box tubing, resting on the brackets (and presumably bolted to it securely.) I don't know if it will fit, but I will measure to see.