Underbed tanks

[summitflier]

Any good recommendations for under bed tanks?

I'm not so concerned with which vendor it came from, but how difficult to install? I have a short step side if that makes any difference. (I was told the bed cross members are closer together than a long bed?)
Many say that "fabrication is required". What does that mean?
Some say I have to notch the cross-member. Any examples of this? Does anyone sell a "pre-notched" cross-member. (Not to be confused with frame notching for lowering). I probably wont go steel, I'm less concerned with weight, but more concerned with rust in 10 years. It seems most are aluminum. Anyone using poly?

How about the gas filler... bed bottom? Rear through the license plates? Side through the fenders? Pros, cons?? I've heard some people run the Jimmy/Suburban tanks, but that they don't fit the pickups without fabrication?

Any pics are helpful.