Mike,
I just installed a crabbing davit on our 16' last weekend. I took pictures to post a how-to, but haven't got that done yet. You can see a picture of the finished product in the Denebola photo album.
I posted the name, #, and address of the place where I got mine out of WA. in a thread called crabbing davit in the Cockpit & Accessory forum. The davit I bought also has a pin lock for the in-use position and for the travel position as well. He will cut to fit the height you want and he sells with a motor bracket and has B&S or Honda motors. I just got the manual labor style for bay crabbing.
I did screw mine to the floor with #14 lag screws and it went in tight. I had asked Les about this and he recommended screws for the fact that if you ever want to replace / remove it, you can. I did use marine grade silicon to keep the water out.
I used the "plastic lumber" for a base as well. West Marine sells it as Starboard but I found the same thing for much less$$$ at TAP Plastics. I used a 1 inch thick piece for the floor which I shaped to give a flat top side, and a 1 inch piece to bring the gunnel mount out far enough to make a perrrrrrfect fit. Tap Plastics calls it C-Board, which I thought was totaly CDish

. They will cut to size, although I bought a scrap 20"x10" for only $10. you can cut it with a regular hand saw and then I used a wood rasp to shape it from there. I hear you can use a small hand plane as well, just keep the blade depth shallow.
Be sure to put the gunnel mount on first and then slide the post through to see exactly where it will fall so that it won't bind on you. I made the mistake the other way and had to move my base a little. This meant more holes in my boat of course :cry :cry :cry.
Okay, I guess I've just about written the how-to so I will post the rest of my pictures on the Denebola web site so you can see some of this and when I do the actual how-to, I will have one of the nerd gods move it into the library.
I'll put pictures on now. Jon