Help ID Aftermarket Gas Tank, Square Bottom, Side mounted, Photo Included

[Maximus]

Anyone know who or where this gas tank may have come from? This blazer sat untouched for the better part of 10+ years by a Previous Owner, so its not a "new" tank by any stretch of the imagination...

Tank has a flat bottom, a drain plug on the REAR passenger side of the tank (not bottom) and is mounted with side brackets.

they seem to have jacked up the venting system (if tanks full on a hot day it will overflow fuel from filler neck) and the sending unit doesn't work. debating trying to fix problems with this oversized tank or just go another route.
Biggest annoyance is getting the sending unit in the Blazer working, and to be fair I have not crawled under far enough to hit the back of the sending unit with an Ohm meter... but if it has failed it would be nice to know what I have so I can try and find a replacement...

Also side question, if this tank doesn't have a vent should I consider a vented gas cap? I didnt see a vent on the sending unit, maybe theres one by the filler? But i know its not vented now as it overflowed in heat...

You guys seem to know a TON about these trucks, so here's todays challenge lol...

Bottom view:

Filler neck from left side of frame:

Abandoned vent lines? shot looking up towards filler neck on quarter panel:

Front side of tank looking at sending unit configuration and location:




So.... ANY ideas?