How to Lower the Ouptut of Oversized Furnace
If partially closing a single supply vent caused your high limit to trip you probably have an restriction problem.  

I would recommend investigating a bypass damper with a return and supply temp. sensor.  These devices are typically just a part of a whole house zoning system, but can be used to easily resolve supply air restrictions in situations where modifying existing duct runs is not practical.  Essentially, the bypass damper dumps excess air back in to the return.  The two sensors ensure that you don't exceed safe operating temperatures without reaching the max temp of the high limit in the winter or freezing the system up in the summer.  The only brand I have used EWC Controls which work just fine, but there are other brands available. 

footnote: reducing the static pressure of your system will reduce energy usage.