No two RV-8 installations are exactly the same. With harness lengths or LRU location. I would start with a plan view of the RV-8 you can write on. Mark each LRU location on that drawing and then plan which two LRUs will be the ends of the CAN Bus. The shortest overall CAN Bus length the better. Then you should consider doing a rough schematic of how those components will be connected to each other. Not a detailed schematic, but rather boxes representing each LRU and one line between them for each interconnect harness, not every wire in that harness. Your harnessing supplier should be able to provide you with a schematic or a spreadsheet showing the interconnects. The plan view can also help the harness tech so send it to them when you know the lengths. Write the lengths on the plan view. Also cross-reference the sample wiring diagrams in the appendix of the installation manual. When you nail down the exact position of each component throughout the airframe, you can run string through the airframe the same way the harnesses will run. You can mark the ends with tape or similar. Then measure the string when removed and straightened out. Repeat for each harness run. I would add 10 to 12 inches extra for service loops or unforeseen routing diversions. If you instead "guesstimate" the harness lengths then add more length yet as you don't know what you will find when it comes time to route the harness.
It probably doesn't need to be said but for the CAN Bus make sure the harness shop uses the good CAN Bus-rated shielded wire instead of regular aircraft twisted-pair wire.
Walt is correct about the Garmin autopilot. He has pulled out a lot of T, D, M, A and G autopilots to swap with GFCs. It might seem unnecessary since you already have the Trio. If it were me I would hurt once and swap to GFC and have a personal airliner for decades. It's your call but do yourself a favor and fly with somebody who is proficient with the GFC so you can compare. If you decide to go with Trio for now and later decide to go GFC that's an option but that will require a complete tear down and extra harness work at that time.
