Achipmunk comments, "...outside frame tank...not enough room."
You did not use a big enough hammer!
This time next year my panel will be a rolling frame. I will install the tank then. I am quite accustomed to slicing and dicing frames and crossmembers. That tank will be a comfortable snug fit. Between the 19 gallon and the 13 gallon, I should be able to make it from Southern California to the Bunny Ranch up near Reno on one fill up. Dropping in an old three speed with a pull cable overdrive to get there quicker.
Most unique job I did was shortening a 1958 VW Van. Took out one of the rear side doors, shortened that puppy about two feet, installed the extra door on the driver's side. Slid a Corvair six cylinder up its backside. Real eye catcher.
Billyray