Section 8 of the factory service manual has a number illustrations to help you. Best bet would be to start with a review of that section and see what your truck has and if it is stock or has been modified. Your question is worded such that it seems you are not at all familiar with the generaal system much less your variation. The seat back if not the whole seat really needs to come out. The hose extending through the floor is the feed line from the tank. It originates at the sending unit/fuel pickup. Don't forget the vent system too, but being a C20 you probably don't have the evap. recovry system to worry about.
If you haven't you can download the factory manuals here.
http://67-72chevytrucks.com/vboard/s...d.php?t=558016
Finally there is no real need to replace your lines beyond a quality rubber hose. But I by choice use a high pressure rated (fuel injection) rubber hose when possible it just seems to work a lot better.