I can't point you in a direction of how, its just something I would do.. I've mended broken tanks before. And yes I personally would remove the rest of the broken piece and then put a piece of clear 3/8" acrylic and probably 2 or 3 inches wide back in its spot. Even if you don't do anything with it your still getting a steal.. 360 or 480 gallons for 275.. Thats less than a buck a gallon. I'd buy them as long as they all held water. Heck I'd probably buy them even if they didn't as long as they didn't have any structural issues with the glass and repair them if they didn't hold water. But thats just me I like to DIY stuff.
With how old they are however before you try and use them I would also replace the silicone on the inside joints. (don't take the tank apart) just razor out the part in the corner and then replace it all.