I'm extremely conservative with risk taking, so I'm projecting those values on this discussion.
Those dimensions would be a 360 gallon but with water displacement and lower filling capacity that could be 315 if that's what they meant. Is the tank 24" tall or 36" tall? The "315 Tall" makes me assume it's taller, than wide. If the tank is 36" tall that tank very underbuilt. Walk away. At 36" tall it should be at least 1.25" thickness. That would explain the crazing and stress on the seams. At .75" thickness it's over a .5" thickness too thin and showing generous signs of stress from the photos. If you take into account that most acrylic brands, except the most expensive (polycast which I doubt this tank is made of) are really .708" and not a true .75" you just keep digging deeper. The tank looks taller than wide in the photos so I think the assumption is correct. Yes solvent welding gussets with 1"ish rods could help, and tip and pour with weld-on 40 could also be a benefit for integrity...but there's an issue that's easy to overlook.
If there is already weld on 16 pour job, then your gussets aren't going to fit flush. Why does this matter? Even if you try to weld acrylic rods (gussets) they won't bond well because they are trying to bond to the uneven surface of the weld on 16 instead of the interior seams and trying to bond to this less responsive material instead of cell cast acrylic that the tank is made out of. It won't give the "bite" plus the weld on 16 job has rounded the interior seams and the rods have square, 90 degree corners, can you visualize? It won't stick as well, or fit flush, sure it will help but not nearly the level of support a flush weld would provide. Not to an extent to overlook it being .5" too thin. The person that poured 16 messed up and compromised any future job you can do. IMO
The panels rarely fail, the seams are nearly always where the failure occurs on acrylic tanks. That's where stress matters most, not in the panels. Crazing and white spots (separation or "dry seams") in the seams is far more important than crazing in a panel IMO. I've seen tanks with great seams and crazing in panels that didn't concern me. I've seen tanks with ugly seams and clear panels that I wouldn't even consider owning.
Weld on 16, this product is garbage IMO, nearly worthless for structural support. Only use as a gap filler or sealer, applies terribly and nearly always leaves bubbles, cheap mans #40. There are other acrylic building forums that go on at length about this product and how it doesn't have a place in acrylic tank building anymore.
If the tank is 24" tall you have a chance. But 36" would never be allowed on any property I own and none of my stock would ever go inside, I wouldn't sleep well.
2 cents. The Diesel says walk away. But you are free to decide for yourself. The only thing I would put in that tank is a lizard.
