Even when you're using Late Binding you still need to provide all of the parameters in the same order as the original declaration.
expression.InsertCrossReference(ReferenceType, ReferenceKind, ReferenceItem,
InsertAsHyperlink, IncludePosition, SeparateNumbers, SeparatorString)
If you aren't using a parameter then you can replace it with EmptyParam.
So I think your code will be:
Selection.InsertCrossReference('Heading', 7, '49', True, False, False, ' ');
(I think that wdPageNumber's value is 7).