1980 Ford F-150 fuel tank repairs

Alright, we are trying to resurrect a 1980 ford f150 2X4 that has been sitting 10 years. We have read articles and managed to start up the engine, but we had to hook up a lawn mower gas tank to make it start, as we weren't sure as to whether we could use the normal gas tanks or not. There was gas in the tanks when it was parked. We think that the gas may have evaporated, but we aren't too sure-we have heard that when gas sits, it turns into a sludge that can seriously mess up a engine. If it is a sludge, is there something we could put in the tanks to help dilute it so we could drain the tanks?