First project on the boat.. New livewell.. Could use some help..

Your going to have a tough time finding one already made that will fit a big flathead so your probably going to have to get one fabricated.  You might know someone thats pretty handy with a mig welder that can put something together for you.  Also take into consideration that water weighs 7.7 lbs per gallon and anything with any size is going to throw the balance of the boat out of whack.  I just redid a 17.5' tracker and had a tough time situating 2 group 27 batteries for my trolling motor, had them in the back at first and the boat wouldn't get on plane very well.  I moved them forward and the boat wants to plow with 70 hp pushing it.

Best advise is make a mock up of the live well you want out of cardboard. Take the time to make it exact, because when you go and have one built you can give them the mock up, so there is no guess work.
 
Cardboard is cheap and you can take your time making it exactly what you want, make sure to include the holes you will need in it.
 
By the way water weighs 8.35# per US gallon. witht that you can figure out your volume of your mock up and see how much weight youll add if you had the live well half full, or full (no one fills it all the way keep that in mind)

I stand corrected water weighs 8.35 lbs per gal.  (looked it up!)   Probably 40 years ago I recall a teacher telling me gas was 6.6 and water 7.7.  I just used regular 5/8 plywood for the flooring but took the time to seal it with a layer of fiberglass mat and epoxy resin, figured it would last longer than what manufacturers put in boats back then.

I got the stuff I used for mine in mars too. No complaints. I took all the carpet out of mine when I redid it and just painted it with marine bed liner type paint. It doesn't look as good as carpet bit for a guy that wants to focus on fishing instead of cleaning I'd highly recommend it. Just take your time redesigning it. I semi rushed mine and now wished I'd have made more storage and stuff when I had it apart. I don't think the bigger live well will be too noticeable weight wise. I know my Lund is a toad regardless.:)