Where the new tunnel has a flange you should drill holes and tack it in, if there are areas you can butt weld that is good too. The problem with leaving 2 in. for error is that 1 1-1/4 excess metal will protrude into the trans tunnel underneath. That may interfere with the trans, you may cut yourself on it later, the bare edge will collect dirt, invite rust, ect.. just measure 2-3 times and cut once. You can also peen up the edge underneath and spray it with an undercoating. Use a "Paintable" undercoating as it will get hard, the other stuff never does and is too soft, IMO.