Local knowledge is king with this one. Have you seen what the other boats do on the same bit of the river? You should be able to fit the cover if you add extensions to the straps so you can go around the boat to pass them under but they will get horrible and slimy and will rot eventually due to immersion. Do not forget that water will still get in to the boat down the mast etc so you need to visit the boat periodically. Are your bailers completely watertight (if you have them)? If mooring for any length of time then anti-fouling will be needed, expensive, time consuming and difficult to remove in the future. I would never moor a woody.
Dave.