Don't fool around with the tank mounting, if you ever get a DOT inspection you'll have your hands full!! You can buy the proper stuff from ANY truck dealer (Mack, Frieghtliner, Sterling etc)...
Basically you'll need 3 items (in pairs)... A pair of tank brackets, if it's a square tank then they will be "L" brackets, if it is a round tank it'll be a quarter round bracket. Next would be a pair tank straps, and to go with that you want a pair of tank strap insulatores (rubber strips that fit the strap.
Tank straps will look like THESE THINGS
except just get plain black ones, chrome ones will be very costly!!
The end product would like something like THIS
The bracket holds the weight, the straps hold the tank to the bracket.