I hope this helps. Shout if you require better photos of a specific area...
The rear area is simple - it consists of a roller on each side containing a single run of the webbing. This webbing runs inside the U-channel of the hoist to the front part, where it is fitted to a dedicated roller on each side.
The front part is a bit more complex. You'll end up with two rollers on each side. One roller lives inside the other end of the U-channel and guides the webbing that comes from the rear of the hoist. The other roller lives inside the top part of the fancy 45-degree bracket and handles the webbing for the front part of the hoist. The 45-degree bracket is bolted onto the inside of the U-channel. You'll have a similar setup on either side of the hoist to handle the front and rear of the webbing:
The bottom part of the 45-degree bracket contains the shaft that actually performs the lifting. You'll fit the rear and front webbing onto this shaft, and repeat it for the left & right sides of the hoist. The webbing going to the rear roller is obviously much longer than the webbing that goes to the front:
A shaft connects the bottom of the two 45-degree brackets together so that both sides are lifted by the same amount when you crank the handle of the worm gear.
I found it easiest to construct it on the ground first. I took my measures like that, then I welded some flat bar to the underside of my roof, drilled holes where required (the U-channel of the hoist is pre-drilled at appropriate points) and bolted the hoist onto the flat bar.
Christo (the snake man)
1998 4.2 GL Patrol (Chuck Norris)
2007 350Z twin-turbo coupe (Batmobile)