I didn't fill any holes on the firewall. I did take body filler and filled in all small dents and dimples leaving the firewall having a 'smooth' appearance. Where possible I recalked all seams and smooth those seams before the calking dried.
Get yourself new wiring harness. Cover all wiring with Braided Wire Wraps. On the ends of the Braided Wire Wraps use heat shrink tubing. That will keep the ends from fraying and it's visually more appealing. Try to hide wiring as much as possible. My headlight wiring is behind the left fender and over the wheel well. Washer hose follows the same route. Hide the wiring for the right headlight under the top portion of the core support. The power lead from the alternator, one wire, runs over the intake manifold, behind the engine, to the positive cable on the starter. All leads for the EFI come from the back of the engine and over the intake. All wiring is covered with Braided Wire Wraps and are tied back with black plastic zip ties. Use the shortest/smallest ties possible to keep it visually appealing. Clip the zip ties as close as possible for neatness and to keep from cutting yourself on the clipped ends. Get good spark plug wires and run them from the distributor, behind the engine and preferably under the exhaust manifold/headers. You might have to get a set that you need to 'cut to fix'. If you do spend the money on a good spark plug wire crimper. There are no wires hanging off of my engine.
I'm not using the factory junction block located on the right fender that is normally connected to the battery via a fusible link. I have a six terminal buss connector mounted on the core support, under the battery tray, that connects to the positive battery post through a 40 amp fuse. That is also where relays for my electric fans are. This allows for shorter wires, the shorter the better when carrying high current to the electric fans. All my power for the truck, except starter lead, comes from the buss bar and is run behind the right fender and over the wheel well. From where the fender meets the firewall the power leads travel under the heater/AC housing and enter into the cab there.
Be consistent with all nuts and bolts. I used stainless steel almost exclusively that I hand polished. Be creative and hide as much wiring as possible. That might require modifying harness somewhat. Just use good connectors and the proper wire gauge.
Remember just to take your time and think through all options.
Taylor Diamondback Shielded Battery Cables 20238
Taylor Diamondback Shielded Battery Cables 20224
Summit Racing® Braided Wire Wraps
Here is another example of hiding wires on my 67 Cutlass with a 468 BBC.
