WPF twoway binding of array elements

I have an array of values representing columns widths. I am trying to bind those values within a grid such that the displayed column widths match the array. The problem is, I can't use explicit indexing (e.g. [0], 1) since the number of columns will change dynamically. I found this question which suggests using a MultiBinding to let me use dynamic indexing. This gives me the following code when a column is added:

while (mGrid.ColumnDefinitions.Count < maxColCount)
{
    ColumnDefinition colDef = new ColumnDefinition();
    int newIdx = mGrid.ColumnDefinitions.Count;
    if(ColumnWidths.Count > newIdx)
    {
        if(ColumnWidths[newIdx].Value < 0)
            ColumnWidths[newIdx] = DefaultColumnWidth;

        MultiBinding bind = new MultiBinding();
        bind.Converter = mHeightWidthConverter;
        bind.Bindings.Add(new Binding("ColumnWidths"));
        bind.Bindings.Add(new Binding { Source = newIdx });
        bind.Mode = BindingMode.TwoWay;
        colDef.SetBinding(ColumnDefinition.WidthProperty, bind);
    }
    else
        colDef.Width= DefaultColumnWidth;
    mGrid.ColumnDefinitions.Add(colDef);
}

with the converter:

public class HeightWidthConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if(values == null || values.Length < 2)
            return null;

        ObservableCollection<GridLength> array = values[0] as ObservableCollection<GridLength>;
        int index = (int)values[1];
        if(array.Count > index)
            return array[index];
        else
            return null;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)    
    {
        throw new NotImplementedException();
    }
}

This works well for oneway binding but I need twoway binding. Looking at the ConvertBack function of the convert, I can't figure out how to get that to assign "value" to the correct element of the original array.

Suggestions?