Saving Old Seagulls

Am about to begin some barrel sailing. Have my Gull set up in barrel of water for final tuning before mounting on boat. Are there any special precautions in running motor in barrel? How full can barrel be (how much over exhaust outlet); run with gears engaged?
Full throttle? If so for how long, etc.?

Many thanks! Wife wants motor back on boat and back on water to get rid of what she calls "the stink" behind the house.

Some say remove the propeller, but as you have the luxury of a clutch, you don't need too.
However deep in the water you have the motor on the boat, repeat that and you shouldn't go wrong.
Run as long as you wish, as long as its cooling, rev in gear, how high depends on how wet you wish to become.

Keith, many thanks for your help through my last few posts.

Did a test run, with water as deep as you suggested. After number of minutes at moderate throttle, I noted cooling water coming out of block was getting hot. Eventually got hotter than I could comfortably hold my hand in. I shut down motor and exhaust tube and block were sizzling hot. Reflecting my recent experience on my boat when motor slowed down and stopped, but started easily when on trailer minutes later leads me to think the block may be cracked a bit and when engine warms up crack opens and lets water into cylinder? Maybe? I have another block of unknown provenance and could perform major surgery and swap blocks further mongrelizingthe motor. Observations?

In the meantime I am ready to barrel sail my similar vintage Johnson (1960's? will look up serial number).