Welcome, I would make a base plate with a couple of pins to fit the winch mount and put 6mm R clips through that way you can put it away if you have a wash down.
Or bit of studding welded from the back and bolt it....
The weakest bit is the pulley mount, I had a pit prop brace under my hand pulley as Dad could only afford 3x2 when he built it but I made a span from a Acro prop from the wall plate to a 7x3 centre brace.