Decks, Patios, Porches and Docks - Joists For Boathouse Boatlift

Greetings everybody, this is my first post here! Sorry for the lengthy post but I wanted to make sure my case was clear. I had a question about adding some joists to my boathouse ceiling in order to hang a boatlift that uses slings. I've attached some pics of my boathouse, boat, the type of lift I'd like to install and a rough drawing (see below). I'd like to put the overhead drivepipe of the lift to one side (near the pier) in order to allow more room for the boat windshield. The boathouse is kind of low to the water (I'm on a protected tidal creek off the Chesapeake Bay), and only has 3 beams for ceiling support which are 8x8 (actual dimensions--pretty strong).

I'd like to hang the boat dead center under the boathouse. This would allow me to hang one 'forward' sling directly from the center beam, and one 'stern' sling about 2' from the end beam. The slings would be attached to cables, which would be supported by the middle beam and some new joists. Since the rafters of the boathouse are only 2x6's, I don't think they're strong enough to hold the stern sling. The boat weighs about 3000 lbs, which includes motor, gas, and gear. That means approx. 1500 lbs per sling, which means about 750 lbs per each side of each sling. So basically, there's 4 points that are handling the total load. I tried to balance the boat so that each sling gets half the load. Most of the weight is in the stern cause of the motor and fuel. I used the 30/70 rule, so I figured the balance point is 5.4' from the stern (.30 x 18' = 5.4'). To be safe however, the total load per load point should probably be estimated at 1,200 lbs, allowing for error in balance point and possibility of a couple of people in boat while on lift. Most span tables I've seen deal with live or dead loads for joists in floors. I'm not sure where to go for hanging loads such as for boatlifts, so I figured I'd ask you guys.

I thought I'd need to add some joists for support from the end beam to the middle beam for the stern sling. Since the beams are 8x8, I thought of using 2x8's, one on each side for sling support, and possibly doubling them up if needed. I guess a basic question is what does it take to hold 1,200 lbs at the 2 support locations for the rear sling (see drawing). Again, I'm not worried about the forward sling where the 8x8 beam will be used. It may be possible to use 2x10's instead of 2x8's, but the bottom 1 1/4" would overlap the top of the pilings since there's not enough for them in the ceiling. Also, I'm not sure if I need a 2x8 that runs perpendicular to the 2x8's to keep the joists from bowing inward due to lateral tension from the drivepipe of the boatlift to the pulley on the other side of the boat. Similarly, a 2x10 would only fit if modified--it would have to be notched at the top 1 1/4" where the 2x6 runs flat under the center of the roof (see pic).

So here's the questions:
1. Are 2x8's adequate to support the load? Should I double them up?
2. Are joist hangers sufficient to attach the 2x8's to the side face of 8x8's?
3. Should I scab 2x4's or 2x6's to pilings under joists for extra support?
4. Will basic joist hangers work or do I need stronger/more anti-corrosive type?
5. Is pressure treated lumber required since it's a wet environment, even though it's under the boathouse roof?

Thanks for your time. I appreciate any suggestions.

Here's the pics:
Boathouse From Shore on Flickr - Photo Sharing! (http://www.flickr.com/photos/jaksprat/3422976403/)
Boathouse View of Middle and End Beams on Flickr - Photo Sharing! (http://www.flickr.com/photos/jaksprat/3422976433/)
Trophy Side View on Flickr - Photo Sharing! (http://www.flickr.com/photos/jaksprat/3422377656/)
Sample Boatlift With Slings on Flickr - Photo Sharing! (http://www.flickr.com/photos/jaksprat/3423783144/)
Drawing of Boathouse and Added Framing for Boatlift on Flickr - Photo Sharing! (http://www.flickr.com/photos/jaksprat/3422976391/)

Thanks!
Jack