Added:
Crossed posts with @dmr. I think we are both saying the same thing - you will fit if you are both clever and patient!
The critical technique is being able to tuck behind a closed gate going up, as that gives you an extra 2 feet or so, but all the Rufford locks are wide so it's fine. You will get wet going down though - make sure you don't let too much water in the back of your boat!
Going down, just take your time. The cills are rounded not straight, so you need to to have the bow in a corner and the stern in the middle, but without snagging on anything. If in doubt, close the paddles! For what it's worth, I would do it in a 63ft boat, and if you are doing it before the middle of May drop me a PM and I will come and help if needed.