Last month I used the boom to lift
an old Volvo MD2B
out. That engine is at least 400 lbs. It didn't seem to stress the rig too much at all.
I did put some some wood block on top of the boom in an effort to spread the load. I tied the main halyard
to the end of the boom. Lifted the end of the boom up to where I needed it. Lifted the engine out with come-a-long/puller from harbor freight. Placed the engine in the cockpit
on some 2x8 boards laid across the benches. Reseting the system to move the engine out over the side involveed clearing a path to swing the engine out to the dock, using my main sheet/traveler to move the engine out and finally I had to take a stanchion out. Overall it was a pretty easy operation.
My Boat is 36' @ 14000 lbs and 400+ lbs of engine hanging over the side didn't cause any crazy heeling issues at all. I used the same set up to move the new engine on.