I'd determine, on the profile pattern of the original, the web thickness and the rod hole diameter inside the forestock, and the web along the bottom of the forestock, and make your new pattern involving your own barrel with these same dimensions. In other words, you are simply subtracting the difference in the two barrels from the profile pattern. This will affect the trigger area and into the wrist too along the bottom lines of the pattern. Other than that, use the original's pattern as is.
The top view pattern will also be diminished by the difference across the flats at the breech, which in turn will affect the lock panels and wrist.
Hope this helps...