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On a 34 foot Hunter I'd expect at least a 4 part - hopefully a 6 part purchase inside the boom (similar to the mainsheet but smaller line). The aft end of the tackle will be shackled to the wire that comes out the boom end sheave and attaches to your mainsail clew.
You can check how many parts it is by seeing how much line you need to pull to get, say, an inch (2.5 cm) of outhaul travel. If it takes 6 inches of pull to get 1 inch, you have a 6:1 tackle.
By re-reeving I meant you need to run your new line properly through both blocks inside the boom. Since the bitter end of the tackle is on one of the blocks inside you'll have to get at that. This will require removing the rivets and pulling the boom off the end cap.
Rivets can be replaced with the proper tool and appropriate "pop" rivet style fittings. Make sure they are aluminum or stainless steel and of adequate size and strength. Alternately you may be able to thread the holes in the end cap casting and use machine screws.
This is certainly something you can do yourself, and yes, sinking the boat is not something you need to worry about (at least for that project!)