When scuff sanding - I use 220 grit.
Looking at your boat, I believe you can leave the splashwell in. You'll need to take of the corner caps, then you'll need to take of the splashwell cap - it is probably nailed on (and be careful when you do - if it isn't coming out then you probably have some screws/nails/bolts still in it - if you force it, it is easy to bend and break them, and you'll have to fabricate a replacement). Under the splashwell, you'll probably find an aluminum lip which is screwed into the transom wood. You'll have to grind down the heads of the transom drains on one side of the transom, and you can then tap the drain tubes out with a hammer and screwdriver - be careful not to damage the aluminum transom. Then look for every bolt or screw you can find and remove it.
Then, once you have finished, you need to try to lever the transom wood up it'll probably be well stuck - I've used a hammer, a mallet, and a crowbar as persuading tools).
These old transoms have a large number of screws/bolts in them. If it doesn't start to shift with moderate persuasion, go back and take another look - there's probably a bolt or screw left in there.
