You can pick up the barrel style aluminum or SS tanks, straps and all for under 5 bills if you shop the sites and craigslist.
On the other side of it, No, lead does nothing for fiberglass, gasoline does NOT eat it (How many corroded Corvette fuel doors have you seen?), and I concurr with Nordies second reason or just maybe years of neglect. Not a fan of fiberglass tanks, had many tell me with E-15 or now e-85 it'll get eaten up, but really, never seen it, and all methanol or ethanol eat up is aluminum.........
Now, Yeah, the tanks need cleaned. My idea? Go to your local bank and ask for old canvas money/change bags. fill them with as many nuts, bolts as you can find, throw two of them in each tank with a few gallons of diesel fuel and drag the boat many times up the bumpiest road you can find quite a few times. Dig out the bags, clean them, then do it again and maybe even again. That or pull the tanks, but if I'd pull the tanks, I'm puttin sumpin better in, which is MORE money. This is the redneck fix..........forgive me, I'm back in redneck country right now.......
Ray