I'll second that. The glass, metal frame and rubber seal should ideally be kept as one piece, you don't need to take the glass out the frame. If the seal is in good condition then I don't see why you would need to use a new one.
The hole you cut for the rear window frame will be a lot smaller than the frame itself but as Madone says, you trim away the excess once the frame is in place.
And get a mate to help!