Which Tremec TKX Do I Get

I can choose either the .81 OD or .68. With the 3.55 rear axle and .81 OD I can cruise 70@2400, with the .68 I cruise 70@2000. My engine is built for low end torque. It made 465ft;lbs@3000, peaked 510ft;lbs@3900. I can be cruising 60mph@2600, floor it and it pulls really hard. I just want to make sure I don't lug the engine while cruising the highway. I admit I'm a little paranoid about choosing the wrong OD gear. Yes, the engine is in the car and running.