I didnt notice a 20 gal tank in the list. do you already have a tank, is it poly?
which includes some of the items you have ordered. plus extensions, and strap.
0051XL from here: http://www.mtscompany.com/jeep.htm
You will probably need to have some light sheet metal to shield the exhaust pipe from the tank and a way to attach.
You dont need the filler and vent hose (same for 15 or 20 gal) but if they are the original 1978, good idea to change. And probably good idea to change all new fuel and return line. Take care that the fuel line is the correct size.
Do a search on the forum, the exchange as been talked about a lot and there are some good ideas and tricks on remove and install and upgrades.
Edit: Dugh, just saw your comment that the tank was not on the list.