No one makes the 71-4 B body firewalls.
They market E body a/c walls and they are the same as 71-3 B body but they haven't done the research to know this. I have. 74 B is different.
For a 71-3 B non a/c wall, used is the only option.

72 Satellite Sebring Plus 440, 72 Dart 5.9 4-spd, 68 Valiant, 73 W200, 78 D100 sb, 78 D200, 98 DAKOTA, .
Moparmarks Parts & Restorations
Desert Mopar Metal
Grand Jct. CO
970-261-7039
http://moparmark.com/
motormangj@gmail.com