Go through and double check the 24 gallon dimensions to the locations on the boat.
From what I remember the extra 1/2" width on the top of the 24 gallon was the deciding factor for me to go with the 18 gallon so I wouldn't have to cut the gunwale and rebuild that area to make it fit. It was so close, I even made a template out of plywood of those dimensions.
Also seems like cutting the gunwale to make it fit was gonna run into the curb for the backsplash, further adding to more work.

Holding off on getting tanks just got the report from boat mechanic 70 psi on 4 cylinders and 90 on the other two. So its rebuild time.

thats not too bad. What kind of engine do you have? I had worked on an engine (merc verado) that had low compression in cyl #6. I called merc tech for a powerhead authorization because it was still in warranty. He told me to first run a shock treatment of mercury quick kleen fuel additive. So I mixed an entire bottle of quick kleen to a 3 gallon tank of gas and ran the motor on it till it was empty. Guess what. Compression was restored. It was actually better than the motor next to it(twin engine boat). I ended up doing it to the other engine too. It's worth a try. Merc makes quick kleen, Yamaha makes ring free, omc makes carbon guard its all pretty much the same stuff.