The babbitted rods are going to be a lot heavier than the finished machined insert rod, so its going to be difficult to estimate what rod weight you'll need to start out with to arrive at the inserted rod weight. As keep in mind that 60 years ago when these rods were made, the set you have may have been considered to be a matched set and close enough for GM at that time, since they didn't weigh things in grams back then, only pounds and ounces.
