your option might be a bit limited if you would like to keep the tank where the spare would have gone. if you can find a 72-87 ramcharger fuel tank that would probably be the best option. you just need to grab the brackets off the vehicle you get the tank off of. the ramcharger fuel tanks are generally 30-36 gallons. there are also a few years in the 70's where dodge ran dual fuel tanks. it should bolt in but might be a bit difficult to find. the other option would be going with something like a fuel cell.

 

Other tanks to consider...
- 80s/90 F-series aft tank (19gal) or Bronco (30ish gal)
- K5 Blazer/Jimmy rear tank (25 and 30ish gal)

Spectra Premium has a good online catalog with dimensions.

 

You need the fuel door and hoses going to the tank also. Why not put it where the factory did in the later pickups along the frame on the side. If you can find a truck in a junk yard or craigs list that they are parting out you can get everything you need.

 

could just be me but if i am going to go through all the work of moving a fuel tank from the cab and having to do body work minus well move it to the back for better weight distribution. the factory setup is really not ideal but it is what they did. there is a reason the leaf and coil springs on the drivers side always sagged and people always post about it looking for solutions. to me i really like the ramcharger fuel tank setup or maybe a fuel cell between the frame rails at the back.

 

The trucks with a midship tank have a taller, shaped crossmember that it straps to.
The ramcharger tanks are difficult to find. Its great they're plastic, but that means they didn't rust out, so there's no new replacements made.

I believe you could go as far as 40 gallon. There's a retrofit ones made for the Fords, plus stock Suburban tanks. Ford has the same width frame as Dodge for many years. GM is narrower.

 

i know the ramcharger ones are kind of rare. that is why i also suggested a fuel cell. pretty much anything that would go between the frame rails at the back would be a good option in my opinion. the only thing it might do is lower how much weight you can put in the bed before it start dropping but to me something like that wouldn't be a huge deal. not too many people really work trucks this old too often. even i only really work my truck a few times a year.

 

If you move the tank from cab you will need to close the hole from the gas filler anyway. Bodywork!

 

but if your going to go thru all that work minus well move it to the best location for a fuel tank instead the crappy stock location used on the newer trucks. plus if you use a fuel cell you could possibly even plan for future mods and get 1 that you can get a fuel pump for so if you decide to run fuel injection you could.