My 81 B250 Sportsman Custom has the large plastic tank.
One thing to consider which hasn't been brought up among all the really great advice so far is that the 30+ gallon tank occupies space where the rear anti-sway bar would fit, so you can't run a stock rear anti-sway bar with the large tanks.
The large tank extends all the way up to the frame crossmember over the rear differential. It is also a much deeper tank. The hump on the bottom which Astrocreep66 mentioned is nominally where the pickup tube filter sock should end up.
Also, I found that the automotive parts chain called Auto Value has the 2" tank filler hose. I just replaced mine this fall. It's $25 per foot, but it's Gates hose; good to go!
If you replace the small breather hose running parallel to the filler hose, be *really* careful you don't kink it where it feeds through the frame. The fuel will spit back out at you during fill ups and shut off the gas station pump if that hose is obstructed!
BTW, if you live in a really rough neighborhood, I've heard of kids stabbing the plastic tanks with a screwdriver to steal gasoline...
-It's been such a LONG TIME... BlueShift>>
1981 Dodge Ram B250 Custom Sportsman Maxi Van
It's what you learn after you know it all, that counts...
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Learning from my own mistakes is good, learning from yours would be much better!