Based on several other examples, I decided to fabricate a 2" wide 'sub-panel' that sits right under the original panel. It will house the throttle, mixture control, carb heat, and ignition switch. I may possibly also add some switches too. This frees up the relatively small original panel for instruments only. You can see from the mockup what I'm contemplating, but in truth an IFR GPS is not something I want to invest in right now. The sub-panel was made of 0.050" 6061T6, bent up in my trusty homemade bending brake. It stiffens things up considerably when cleco'd in, and the rear-shot photos show how I've temporarily fitted it to the original panel for drilling. The last photo shows how I've captured the original firewall reinforcement square tube, to maintain the structural integrity of the firewall and panel.
Next step: fabricate an avionics frame to accept the various trays for the avionics.