Here's something that I found posted on another forum for what it's worth:
Chrysler built 3 versions of the 400 for the 76 Cordoba.
A 2 barrel model had 175 HP and 300 ft lbs Torque.
A 4 barrel model with lean burn was 210 HP & 305 Torque.
A 4 barrel model with factory dual exhaust had 240 HP & 325 Torque.
All had 8.2 compression ratio, 4.34 inch bore and 3.38 inch stroke. The 400 was introduced in 1972 as a replacement for the 383 which had been around since 1959.
https://answers.yahoo.com/question/index?qid=20070430234455AAdBXxV
So, I would guess that the N was a lean burn 4 bbl, and the P was the non lean burn 4 bbl with dual exhaust.
