1966guy,
All the gas tank kits that move the tank to the rear will require some fabricating and modifications to your truck. All the reasonably priced kits I have seen use the "blazer" style tanks and they hang down and are visible from the back of the truck once completed.
I didn't like that look so I went a different route. FYI: pictures and data, go to post #8 of my build thread
http://67-72chevytrucks.com/vboard/showthread.php?t=363509Sending Units: Be sure to get one that is 0-30 ohms if your are using your factory 1966 gauge.
Filler Necks: Really depends on what tank you end up using. The ones that mount to the floor of the bed are the easiest to install. But then again if you are hauling something in the bed access to the fuel fill can be problematic.