I know of no place that you can download a 71 manual for free
But you can get a '70 here from MyMopar:
Only problem with the "MyMopar" manual is that the page numbers do not follow the Mopar "dash" system, so you'll have to "play" with the numbers
http://www.mymopar.com/index.php?pid=31
http://www.mymopar.com/downloads/servicemanuals/1970_Plymouth_Service_Manual.zip
Or you can download a 72 manual, posted by AbodyJoe
http://www.abodyjoe.com/pictures/Misc. car info/1972 Plymouth Chassis Serv Man.pdf