Okay here's what I'm dealing with:
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The crack is about 12" across and goes up 2 sides of the tank an inch or two. The area in the yellow circle is tricky because it's some sort of indentation. And then there's trying to get an area clean enough for the resin and ABS to stick. Whatever they used appears to be pretty good stuff - it's like a rock. I ground it down the best I could without grinding through the tank. I'm hoping if I go wide enough of the messy area that the Plasti-Mend will stick.
I looked at new tanks and came up with a couple that I could make work. No exact fits, though. It would be a couple hundred bucks and a whole lot more hassle to swap it out, but I may have to go that route if the Plasti-Mend doesn't work.