Yes ---the g wagon and the w123 flys are the same part, just some earlier ones are off balance matched--You got the engine-fly still bolted together--No problemo.
Also the 37 lb fly we are talking about is also used in the 207,209-307 308 trucks with the 616--it gives more momentum starting in each gear.

You still got to do your due dilligance to ensure you dont get vibrations with the drive shaft--do your vibration problem searches.