The easy way is to remove the part of the locking mechanism that is attached to the lid 'the striker' sometimes it will 'pull' the lid to the left or right on latching, now lower the lid into the opening ,if the gap is not even from side to side open the lid place your hands on the foward side edge of the lid approx level with the hinges on the side with the smaller gap,now give a firm quick push, slowly close the lid and check the gap, not enough do it again ,to much push from the other side.
Now lets presume that the gap is even, re-install the striker making sure that it does not 'pull' the lid on latching, with the standard amount of adjustment you should be ok, but over 45 odd years ,accidents , different lids, sometimes you need to elongate holes to get the disired amount of adjustment. Good Luck.
