Here is to typical set ups of the Norlin Mk III, the single skin with inner moulding covering the entire cockpit area and the sandwich set up with a cut down inner moulding.
These are not the only configurations that has been used through the years but they pretty much represent the better and the worst. The single skin/full inner moulding is of cause the worst, both heavy and soft. Through many years there where no bulkheads in the boat, just covers for the floatation foam so there is no serious structure taking the loads from the rig and keel.
It's wise to be well protected when you hit the boat with the grinder.
Ripping out a full inner moulding saves you around 17kg that did not do any good anyway. Building a serious structure will put back 8-9kg but these will if well executed completely change the performance of the boat.