I am running a 16 gallon 'cell underneath my bed...same size tank as the stock one the truck came with.
http://www.73-87.com/7387garage/chas...ellinstall.htm
If I could do it all over again, and money was no object, I
might use an aluminum one. But to be honest, since this 'cell is underneath the bed, I am very happy with it and don't plan to change it out.
The problem with plastic fuel cells is they need to be mounted so they are not exposed to direct sunlight/heat. If hey are simply bolted to the top of the bed, they will warp and look like crap over time.