Firewall penetration
I have an I0-360 but with a Catto fixed pitch prop.
The throttle cable hole location as per the plans is good and the mixture cable hole location per the plans is workable. On both these locations I used the Van's P/N: EYEBALL SE961-125S plus the 3M 2000 degree sealant.
I have run all my earth wires back to the firewall and I have used the BandC Aero ground block P/N: GB24 (i.e. 24 tab). They have a number of other combinations also available. This allows firewall penetration plus a common earthing point on both sides of the firewall.
For my electrical wiring I have used the firewall kits from Aircraft Spruce. They have three sizes which are 1/2" P/N: 09-02601, 3/4" P/N: 09-00978 and 1" P/N: 09-00979. The size you use and the location through the firewall will depend on the sizing of the wiring bundle and what components are being connected either side.
Before you start randomly drilling holes in the firewall work out where all the components are located (i.e. battery, contactors, etc) and make sure that components such as magnetos do not impinge on the wiring looms from where they exit the firewall.
Good luck with your building.