First of all, welcome to the board. No better place to learn about these engines.

Something does not add up. What RPM are you turning now @ 70MPH with everything locked up? For reference, I installed a 6.2 in a Buick Roadmaster with 3.23 gears and 235x15 tires (28.8 in diameter) and at 70 MPH the engine in 4th gear, locked up, was turning a little over 1,900 RPM. Never had a problem with not being able to keep it there on the level though partial grades would unlock the torgue convertor occasionally.

To put it into context, this was a 5,000 lb. car. Granted the aerodynamics were better than the Rover but the engine had more than enough umph to keep it at highway speeds. Ideally, these engines like 1,800 RPMs, and get their best torque and fuel economy at that speed. Thus I usually drove it at 60-65 MPH.

If I was in your shoes, I would be looking at the current powertrain before investing in another major engine swap. As I said, something doesn't add up.

Bill