Cleaning my gearbox

Before the my new box goes on I was going to give it a clean. What's the best way to do it? I was thinking about using my compressors paraffin gun loaded with engine cleaner and giving it good spray and brush. I will be blocking up the final drive end where the prop shaft goes and I have other neutral and reverse switches to go on, so I don't have to worry about damaging the old ones. Would this be OK?

Cheers

Dave