Best replacement behind-the-seat fuel tank

[DarkFlounder]

The PO of my '72 K2500 removed the behind-the-seat tank and put in plastic saddle tanks.

I'm replacing the bed, and rather than cut holes in the nice sides for the fuel filler, I'm planning on putting a tank back in behind the seat. The filler cap/neck are still in place (just not attached to anything).

So, new reproduction or used tank? Who sells the best reproduction tank? What should I look for/avoid in a used tank? What's the best method for ensuring a long tank life (pour-in tank liner?)

I only plan on doing this once, so I want to do it right.