Start by downloading a service manual
http://www.forabodiesonly.com/mopar/showthread.php?t=244981
http://www.mymopar.com/index.php?pid=31
Get as close as you can as there were changes. If there is no 71, get the 72 manual. Look in the fuel chapter, and in the emissions chapter. I believe your car still used the in-trunk vapor separator, not sure.
There will be two metal lines from the tank to front. The larger is fuel, the smaller is not only part of the emissions, IT IS the only vent. Make certain it is not plugged. Look for a crimp near the engine bay end of the tube. This is an anti--over fill valve, and must be free (not gummed up, etc)
EDIT. I believe the 71 Plymouth chassis manual will be from the "MyMopar" link. This will NOT be numbered in your viewer with the original "dash" page numbers, so you have to "play." Chapter 9A and 14 is what you want ("Cleaner Air" and "Fuel") I usually just type in a big number like page 300 and go up/ down by 100, until I bracket the chapter, then up / down by 50, etc, and then add or subtract
"Cleaner Air" starts page 551 (9A-1)
"Vapor savor system" is what you want, page 9A-6, so just page down
There is more info in the Fuel (Chapter 14) section under "Fuel tank" page 709