Greg,
It's not my contraption. Just something I saw on the beach. However I think you can guesstamate the lengths. The best way might be to pin your mast and determine the lenght from the back of the boat to you spreaders as the chock would need to start below them. You also have to account for clearance at the back of the boat so the arm would be able to swing down without anything in the way.

Remember this thing gets it about half way up. It doesen't put it all the way up. Half isn't bad though. And the last and most important thing is the boat is backwards on the trailer.

I believe the G cat's mast is raised at 90' also. It would most likely stay that way easier (larger flat surface). Of course if you make the chock a little wider and flat it should want to lay at 90.

One other thing about it is the plate's that make up the piviot. Notice it prevents side motion at all points, down,up and on the way up. Very important!

I've seen this rig only once. If he comes out again I'll talk with him about it. Don't hold you breath though.

The lever is a wonderful thing.