Removing Engine Using Boom
I've done what you are proposing twice. Once with my C36 and once with a buddy's CS36. Let me offer a slightly different approach. Do NOT use the topping lift as a support for the boom and hang the engine off the boom. Instead, use the boom only as an attachment point for other lines.
Use your main halyard and halyard winch (it can hold LOTS more than 3-400 lbs) as the primary lifting device. To control the angle of lift (fore and aft), run the halyard thru a snatch block, which is attached to the end of your aft reefing line. If you just use the halyard, the engine will have a tendency to pull towards the mast, and you'll never get it out of the cabin. The main halyard is connected to a chain on the engine lifting eyes. Halyard goes thru a snatch block (resting next to the boom). Reefing line attaches to the snatch block, back thru the block on the side of the boom, forward to the gooseneck, down to deck, back to winch on cabintop. Use traveler and mainsheet to control port and stbd.
As you hoist halyard, engine comes up. As you crank in on reefing line, engine moves aft along boom. Remember to put old blankets or towels around the companionway for padding. You can lift the engine straight up thru the companionway, swing it over to the dock, crank on the reefing line to move it outboard along the boom, then lower it to the dock. It can all be done slowly, with lots of control. You will have to adjust the reefing line as you take up tension on the halyard. New engine can be installed using the same procedure.
If your halyard is not long enough to let the engine touch the dock, you can extend its length with another length of line. Attach the other line to the headboard shackle, and pull the headboard shackle up 15-20 feet. The other line would then go thru the snatch block and get attached to the chain at the engine.
From BBS by: Tom Soko, Julandra C36 #659