I do not recall there being a need for the drop tank to be mounted on the lh side. The last -4 I mounted a tank to, we mounted in on the RH side as I recall. Both sides are more than capable of feeding, especially with the boost pump sucking them dry. I can look in my manuals, but am fairly sure it is not dictated one way or the other.
As for weight, I know a couple of hog owners and none has complained about the tanks hanging one side or the other, so I imagine it is not much of an issue.