Having a 58 Cameo myself I can appreciate your situation but there are several alternatives but first you must make the choice of whether it is better to keep a rather rare truck as close to stock as possible or change it so significantly the value is adversely affected. If you Cameo is a rust bucket then you can do whatever you want and probably sleep just fine but if it is in good condition you should use caution when making a modification such as this.
The main concern is the spare tire carrier is in the same space a rear fuel tank would need to be located. Putting a fuel cell in the bed of a Cameo is just too wrong to even consider.
If you have or plan to remove the spare tire carrier then the solution is easier. If you retain it then you will need to consider a side saddle tank solution which is what I am planning.
Side tank or tanks will need to be custom formed by a sheetmetal shop. There is space between the outer bed side and the inner bed side to mount a tank or if you are concerned about side impact issues you can design one that can be mounted inside the frame rails. This will then likely require some rework of the exhaust system for clearance.
If you remove the spare tire carrier the cheapest and best tank option I have found is a 16 gallon steel unit from Tanks, Inc. The sending unit Ohm range may be an issue but you have not specified what you now have which if stock should be 0-90 ohm. Tanks, Inc should have a sender with that range if not you can find a universal sender to substitute.
If you are going fancy with the truck then there are some more expensive tanks from various suppliers such as Classic Parts, JPL, LMC, Brothers, etc.
Whatever you do I hope you do it right but that path always requires throwing some cash at the solution.