"The tank fills the hole it came from just right."
Really?
I would not use it. The CG is VERY picky about fuel tanks and fule hoses on any permanant installation. I believe it probably has to have a cert sticker on it. Not sure if ABS would take the fuel or not. I believe that the platic tanks are polypropelene and that ABS is a form of polypro, but not sure. I can tell you I've had plastic jugs fall apart from storeing used oil in them to long (what a mess).
If you really want to know contact a tank manufacturer or a plastics outfit. Have you checked the catalogs to see if you can get a drop in that fits?
You need a fill a vent and preferably a gauge to that may not work on that other tank. All the lines need to be a CG class for underdeck gasoline use.