Is/was there ever a 340 Magnum

Magnum name was just marketing.

There was only a high performance 340. No economy version.
Just like only a high performance 426 Hemi.
The Only 2 Mopar engines sizes made in performance versions only. basically factory high performane only motors.

Cross rams, dual 4bbl and such were optional on other engines,

All other V-8's could be had in 2bbl and 4bbl versions.