: once you have built a strongback beam that is straight and flat, so that all
: measurements that come off its top surface at 90deg are true.
: does the strongback really need to be levelled?
: can it not sit on a sloping surface and be moved from one position to another
: etc?
The only reason for it to be level is for your eye. It is very useful to look at the boat to make sure you are keeping things symmetricas you build; especially the the first few strips. If your strongback is off kilter, your eyes might start playing tricks on you.