AZ,
I placed a early camaro tank between the rear frame rails on my 54. I had to move the rear cross member rearward and bend up some new mounting straps. The filler will be through the bed floor.
The question that needs asking and answering is do you have the ability (tools, time, talent) to make the mod. It is not a quickie. If you are able to do your own fabrication then go for it.
If not, you might consider staying stock. If you are keeping the original seat then moving the tank out will not help you much with storage room as you will not be able to get to that area as the seat is fixed.
Having the tank in the cab is not really a safety issue unless your leaks and you smoke. Trucks have had the tanks in the cabs for years with no problems. It is one of the endearing attributes of these trucks to some bolters.
I moved mine so I could have more storage and am using a custom seat that folds forward for access to that area.
Have fun making your decision.
