Thanks for the help!
Cut/dremel all of the internal supports for the stock internals until you are able to close the shell around the blastzooka tank. Ideally, have the stock blastzooka barrel match up with where the stock breech was.
Make room for the pump to come out the back
Figure out how to make the stock trigger hit the blast button on the blastzooka tank (you may need some sort of sheet plastic such as PVC or polycarbonate for this) or attach a separate blast button like what is described in this mod.
Glue the tank into the blaster (or figure out some non-permanent attachment system...polycarbonate brackets if you want to be really fancy, or maybe use epoxy putty to form a sort of cradle).
For some inspiration, check out:
All of the aforementioned references are found in our Modification Directory, by the way...you may want to look there or search the rest of the forum for other "air tank in a new shell" modifications for even more ideas.
And, yeah, probably could have just spent more time looking around, but I do appreciate that some people helped out.