Blazer Tank Diagram--it works! Use this!
 

HEY! So we've had roughly five million threads asking about issues with filling the underbed gas tank. I also have had some real issues (didn't have a vent line, created a fuel fountain on my first drive--bad). SO, now I have it figured out completely. It fills fast, it shuts off when full, the gas gauge works. This is how I did it, and it works great.

You may use different hose/tube diameters, but these are the ones I am using. I bought my gas tank at the truck and car shop in Orange CA. The sending unit is 90 ohm (pretty sure...). Their kit is okay, the straps don't fit that well, but otherwise I have no complaints.

Search terms:
Blazer tank fuel filler plumbing, underbed, gas tank relocate, (mods--hope these search terms are okay, I just want people to find this)

Hope it helps!

Diagram: