For my tank leak (85 cuddy), I turned off the valve on the fuel pickup. The boat was sitting in my driveway on a slight slope so that anything in the bilge would drain to the stern. I plugged it and checked it two days in a row (draining in between) and found fuel in the bilge each time. Since my fuel line was shut off I reasoned the tank must be leaking. The fill and vent hoses are on the top and the tank was not full so there would be no way for a leak to occur there unless it happened when filling the tank.
Sure enough, after removing the tank I found a hole the size of a dime and several other areas with active corrosion that would have leaked soon enough. I am one who bought a 40 gallon from greatlakesskipper on Ebay and it fit nicely in my 85. I have yet to do the final install but it slid right in there.
http://cgi.ebay.com/ebaymotors/BOAT-...spagenameZWD1V