A few questions that will help us with an answer.....
What's the length, width, and tube diameter of your boat? If it's a raft, do you know the center to center distance, or the width of your frame? Will your oar towers be fixed or adjustable? Will you row from a seat, dry box, cooler?
My dimensions on my raft are much smaller than yours, but the issue was similar. Longer oars felt better than shorter ones, but that made the 1/3 2/3 balance a little bit of a challenge. I use an NRS frame, so I got the taller 10 inch towers, and angled them outward to give me more width between the towers. This allowed me to get the balance right. I only angled them as far out as I could without having them go past a level position. Your counterbalance oars should eliminate or at least mostly eliminate the heavy oar problem. If your oars towers are fixed, then you will have to fudge a bit on balance.