If the customer is doing the design there are some important points you need to keep an eye on Andy. First thing is the length of the chain movement. If it's an 8 day movement, for example, the weight will descend for at least that period of time and the cabinet needs to be long enough to accommodate that movement.
The other point is the pendulum swing, the interior of the cabinet must be wide enough to accommodate that plus the width of the pendulum bob plus some clearance.
That sets your width and the height, plus the hood on top, which is why the old ones were so tall.
Hope that helps and the best of luck with a fascinating project.
Roy.