Welding a flange onto the older tank should not be too hard. The older tank will be thicker and easier to weld to than a newer tank. As long as you empty the tank, and make sure there is no fuel puddled in it anywhere (rinse good with water), then blow it out with shopvac, or compressed air for a few minutes, it should be fine. The idea is to get all of the fumes out. I tig welded one into '78 280Z tank and it wasn't too difficult. Some very small vice grips make things easier. Like these:

http://www.harborfreight.com/6-inch-...ers-39534.html