Entering the cabin w/ Dominator ECU (Early A-Body)
 

I have a '66 GTO. I'm switching from a Holley Dominator Carb to EFI. Using a Accu-Fab Throttle body, a Holley Dominator ECU and a Marcella Modded manifold. I'm curious to see where the most convenient spot (on a '66 GTO) is to enter the interior with all of the wiring and where the best place is for the ECU. My battery is in the truck. Thanks to all, but real-world experience preferred. :)

A bulkhead connector is the best(IMHO) overall/long-term solution IF you don't mind the hole. Use a good one. Deutsch makes some nice ones that are sealed from the elements and have good strain relief. The proper tool to crimp the pins is not cheap, but worth it. As far as the battery in the trunk, I can tell your u what I did to avoid running ecu wires all the way to the trunk. I have no rfi issues, or interference at all, so it works. I ran an 8ga wire from the battery disconnect to a relay triggered by the ignition switch(crank/run) and powered a 4 position fuse panel that powers the Ecu, crank and cam sensors(rfi sensitive stuff) The rest of everything is powered from wherever is convenient and ignition triggered by relay also. Grounding is important, I ran a 00 welding cable from battery negative to the back of the cylinder head, and ground everything ecu related there as well. (No ground loops) Hope this is helpful.

Thank You, Scott.
Can you be more specific?
I looked over the Deutsch website and everything seemed to be # of wires specific. I was hoping for a bulkhead fitting that could adapt as I add more "information" to the ECU (Read more input sensors).
47 pin bulkhead and 2 tools (crimpers)?

I believe they make up to 128 pins. I know they make 64 pins as I use one of those. Perhaps 2 of the 64 pins? Keep in mind you can run multiple 5vref together at junctions, engine bay side as well as signal grounds etc. You don't need to run individual wires in every instance.
ETA: maybe mine is 47 pin I'd have to check. But still, you can decide how many connections you'll need and use multiple bulkheads.

Where was the best spot (in your opinion) to place the bulkhead fitting(s)?

One tool should work unless you're going really big on any of the wires. I think mine does 22 gauge through 14. It's been a while I'd have to double check.

I used the block off for the heater core to keep from cutting the original sheet metal. Not sure what you're working with. A place you can easily access it, but where the the wires protected from heat/chafing is optimal.

Here's a pic of the tool. 12-20 gauge.
It works on the other Deutsch connectors as well. They're super nice, but kinda expensive. I intend eventually to redo my coil wiring with those so I can pull a single plug to remove all 4 coil packs per side. Just hasn't been a priority yet...

I still have a heater core and was hoping to find the best spot (real world experience) maintaining said pieces. Motor is out right now. Thought I might save some time rather than setting it back in, taking it back out, etc.
I appreciated the help!

I think it depends a lot on how hidden you want things to be or how willing you are to cut on the car, just a couple of ideas….

To avoid cutting up the car too much think a good location for mounting the ECU is usually up above the heater box behind the glovebox or in the glovebox itself. Then there is a larger factory hole in the firewall where the blower motor wire passes through that can be used to feed the harness through. This still might require enlarging the hole to fit everything through the hole.

If you want to hide the wire harness from view, have also put a hole where the trans tunnel meets the firewall and then run the wire harness under the carpet and had the ECU mounted under the seat etc.

Bulkhead fittings are nice, but not always easy in tight spots. Can also look at Seals-it firewall grommets to just feed the harness through a grommet instead.

In that case, I'd probably use a firewall grommet just above the bell housing area behind the distributor. There are some nice ones available that should allow plenty of room for the whole bundle to pass through.

Considering something like this. I can open up the hole to the correct size for the appropriate bundle of wires.

https://www.mcmaster.com/5302N13/

That would probably work well. Just make sure to get in a place that won't require any severe turns to avoid whatever is under the dash. Heater core, under dash ductwork etc. The amount of wires you'll end up with won't want to make a short quick turn probably.