how to frisco a gas tank?

ok so i searched all the posts i could on this and there wasnt any tech type posts on this so here goes lets see how this works out for me.
i know how and where i want to move the petcock on my tank but the thing that is hanging me up is how to do the tunnel. i havent cut it out yet as i dont wanna mess up my tank. (it's a 70's amc flat tracker tank you know the square side ones) anyways can someone help me out here and walk me through this and maybe post some pics? can i just cut the tunnel out dr do i need to cut out the whole bottom? also when i go to weld the tunnel back in (if i can just cut out the tunnel) will i have to make filler pieces since i have cut the tunnel down? someone please help me here as i just got this bike just after my wedding and now that i am back from my honey moon my goal is to have it all together and running in less than a month. i bought the bike off here from one of the most kick ass dudes i have ever met his name is topher from jersey. please help