Break it down into sections.
The vast majority of instrument panels are NOT structural. If you fly this airplane without the panel installed, it won't break. If the aircraft will break, then your panel IS structural. And no floating panels are structural items. Looking at SJ's panel as an example, it's simply a flat piece of aluminum. Cut it to shape. Mark the forward side of the panel to show where the 'no go' areas are, i.e. any place that you don't want to mount something. Then lay out where the mounting hardware will be installed and drill the holes for them. Now carefully lay out and cut for instruments, avionics, and all other items, such as switches, breakers, lights, etc. Keep checking that each new item fits properly to the panel AND into the aircraft. Keep in mind that if you mount square items such as radio trays, you'll need to allow room for the angles and any support brackets. And allow room for paint or powder coating! That portion of the project is a minor alteration and just needs a log book entry.
Now, any instruments or avionics may or may not be STC'd. If you replace a certified instrument with another like item (also certified) it's a simple log book entry. If you update instruments with something like Electronics International or JPI instruments, it will most likely have an STC on it. Not a big deal. Install it, the mechanic signs off the 337 and the log book, the IA inspects and signs off the 337 and its sent to Oklahoma City. Done.
Moving or replacing the smaller items such as the switches and panel lights are just log entries by the mechanic.
As for the STC'd or certified avionics vs non certified, George hit right with the 430 vs 660. The 660 is just a GPS. The 430 was 'certified' so they put it through a BUNCH more testing. And it has a metric ton more capabilities. You can legally and safely fly the GPS or the nav in full IFR. It will drive most primary navigation instruments available. It will also drive a secondary nav head, if you have need for one. It has vertical and horizontal outputs to couple to most auto pilots. And it has multiple data lines to allow it to be coupled to almost any combination of other instruments/items to make up a custom avionics system. Think about being able to upload a flight plan through a unit like a Flightstream. And if you have more than one 430, they can be coupled together to be uploaded simultaneously. You can input pressure altitude from a serial or Gray code altimeter or output GPS altitude to a unit such as a remote display. You can connect up to air/data computers. You can connect DME systems and RMI's. You can even use it, along with a UAT and transponder, to build an ADS-B system. I'm sure there's more that I'm not thinking of.
Web
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It's your profession Web, but for my own clarification, what do companies like the above I found on the net do. Are you paying for an STC'd panel or can I simply get the local sheet metal work expert to do the same?
I don't understand the whole STC'd expensive GPS thing. What does the Garmin 430 do that the Aera 660 won't?
I'm a round instrument type of guy and have had the above picture saved on my computer as something to aspire to.
Good luck SJ.