As to installing / spraying in a liner the tank has water baffles every 18" or so to stop the water from sloshing so those options are out.
The baffles will also be a pain to deal with when I modify the tank for riders. the front mounted pump will not be removed so it will still be capable of pumping,
i wouldnt worry about the baffles and i would cut them out of the way. most likely your not going to be hauling around a load of water so they wouldnt be needed. depending on the size of your existing tank you could drop in a 100-150 gallon agricultural tank with two connections one for tank to pump and tank fill or just fill the tank from the open connection on the top with a hose and eliminate a connection. as long as the tank is full or empty the baffles dont do nothing. its only when you have less than a full tank does the baffels come into play to keep it from sloshing. cut the top and bottom of the tank out. frame up the bottom to hold the tank and water weight and from the tank from shifting side to side. you can even leave the bottom open. then cover the top of the tank with sheet steel with an access to the tank from the top. the hose in your bed will hide the top side and it wouldnt be seen. depending on your priming means you may need to have some water available on the truck.
i have a project on hold of cutting a access hole on the top of the my tank to gain entry to rubber coat it. at that time the baffles will be removed so i can get access to the whole inside. make a water tight bolted on cover and repack the hose bed. for now my tank doesnt leak.
