As a Scot from Edinburgh I would be a "Lang" man myself. Others may disagree!
I recant, Skoosh, as your post has just reminded me of Laing the Jeweller in Rose Street in Edinburgh and that is pronounced as Ellen has said!
Unless otherwise told by the owner of the name, I would pronounce it to rhyme with 'bang' if it is spelled 'Lang' and 'layng' if it has an i in it.
I have no opinions on how it might have been pronounced by any particular family before spelling became more fixed, other than that the spelling probably followed the way they pronounced their own surname rather than the other way round.