Hey Windsor
The added weight makes it easier to cast and it decreases the float time (time it takes to reach the surface).
It's been a long time since I built crank baits. What I used to do was take split shots and temporarily pinch them onto the hooks and put the unfinshed lure into a bucket of water. When it was near neutral bouyancy but would still float to the top so I had a rough idea of how much lead to add to the body. Then I'd drill into the body roughly enough to be equal of the split shots, and melt lead into the hole. If I added too much lead then I'd use a smaller drill bit and drill out some lead. Be sure to balance out the weight, you don't want the lure to run head down or tail down. It won't look natural. If memory serves me correct I think the added weight had to be about 30% of the distance from the head, as all lures have less wood in the tail, hence less bouyancy. Good luck, hope to see some pics of your finished lures.
P.S.
I used pine as it was cheap and available. I'll have to find my pics.........lol.