Normally it depends on whether we like you or not!
Seriously though, here at KB we normally go on manufacturers length unless there is a good reason not to do so. This could be like having a ridiculously large bathing platform attached to the stern of the boat, or a huge great jet drive sticking a metre out of the back.
Assuming you don't have such issues, I'd talk to your chosen dry stack and ask for a bit of flexibility.
By the way, if 1.1m extra is costing you £600 you must be at a very "exclusive" Dry Stack.