One point about Tonti framed Guzzis is that the seat rails are much lower than most bikes, which means to get enough leg room the seat needs to sit high above said frame rails. You only have to look at any Lemans to see how thick the seat is. That's ok but it makes things difficult if you want to fit a 'classic' cafe seat, as there is a compromise to be made between looking good and having enough leg room with rear sets to actually ride the bike.
In my case I had to mount the Ducati seat unit above the frame rails, so theres a gap between the two.. not ideal .. it looks bad and allows rain water into the electrics box. Also the Ducati seat is a little too narrow to fit over the frame rails, it doesnt really matter as it sits above them anyway, but it still doesnt look right to me.
So the new seat will be made a wider to follow the lines of the frame.. its wider at the back then the front, while also having deeper 'skirts' to fill the gap between the top of the frame rails and the base of the seat. As for the size of the seat hump, I've still not settled on the shape / size yet.