[Andy]

There is not a lot of clearance betweeen the speedo drive gear and the snap ring. If you put something between the torque tube and the rear end, you cut down this clearance. How much clearance you have and the thickness of the new seal should be considered.
I would replace the stock seal. There is a trick to remove the bearing outer race. It is really easy with a simple to make tool. I also made an instalation tool from some conduit and a washer. You can drive the seal out easily but it needs a tool to put one back in.