How can I have a WPF TextBlock that has the same size for all font weights?

I'll jump to the chase: Is there any way to tell the WPF TextBlock to measure itself such that its size doesn't change when its FontWeight changes?

I have a TextBlock that changes font weights dynamically based on a style. The TextBlock is inside a RadioButton so it is Bold when checked, Normal otherwise:

<Style x:Key="BoldWhenChecked" TargetType="RadioButton">
    <Style.Triggers>
        <Trigger Property="IsChecked" Value="True">
            <Setter Property="TextElement.FontWeight" Value="Bold" />
        </Trigger>
    </Style.Triggers>
</Style

and here are the radio buttons themselves:

<StackPanel Orientation="Horizontal">
    <RadioButton Style="{StaticResource BoldWhenChecked}">
        <TextBlock Text="Item 1" />
    </RadioButton>
    <RadioButton Style="{StaticResource BoldWhenChecked}">
        <TextBlock Text="Item 2" />
    </RadioButton>
    <RadioButton Style="{StaticResource BoldWhenChecked}">
        <TextBlock Text="Item 3" />
    </RadioButton>
    etc...
</StackPanel>

Unfortunately, since I am not using a fixed-width font the width of the TextBlock changes when the font weight changes, and the entire panel of radio buttons shifts accordingly, which is visually jarring.