If the rotating parts are already balanced, changing that rod will most likely require a re-balancing job. Also, there is no such thing as a cast steel connecting rod for a 340 Mopar. Matter of fact I'm about 99% certain Mopar never used cast rods in any hi-po V8 engines. Unlike GM, where cast rods could be found in Buicks and Pontiacs.
Here is some useful info from "Enginebuildermag.com":
Connecting rods
Chrysler small block connecting rods are essentially
all the same. And, Mopar connecting rods do have a good bit of
integrity. All are forged, but the 340 rods (p/n 2899495) are
bigger and stronger than their 273/318 counterparts (p/n 2406785).
Both part numbers have full-floating wrist pins. The 360 four-barrel
rod (p/n 3751015) is also a heavy duty forging, but has a pressed
pin. All small block rods have a 6.123" center-to-center
length, but 1974 and later 318 and 360 rods are of the pressed
pin design.
