Contemptor arms in FW Mk. IV shoulders?

[Evil Eye]

I've had a rather bizarre idea for a Dreadnought conversion, using the old FW Mk. IV close combat arm shoulder mount (I am aware they're only available from, er, less savoury providers), but instead of the actual arm, fitting one of the Contemptor weapon arms in. IMO this would look pretty cool, giving a Boxnought the option for an "articulated" main gun. However, whilst the Contemptor arms look like they'd fit in the shoulders with minimal work, I own neither Contemptor nor Mk. IV arms. Can anyone clarify whether they would fit (with or without a little conversion) or if it's completely the wrong size?