Split it and it will seal fine.

You are going to mount the ECM in cabin I take it ?? I mounted my ECM by the battery tray/fender. Only wires going through the firewall are for the DBW pedal and OBD port. Harness runs through inner passenger fender (out of sight)

DT69Cam I have the inter cooler reservoir there, only enough room for the AC and heater hoses to go up by it .
The header is not a problem the hole for the pedal is behind the head almost where the head meets the intake so there's plenty a room.

I have used the grommets from Seals-it with great luck ( use in the offroad world that I deal with also)
http://sealsit.com/product-category/...-grommet-seal/

just a example, they are available in numerous sizes (I.D. & O.D.)

Nice!

We laid out the harness and we decided to abandon the pedal location. the harness must be designed to come out on the passenger side for the injector connections to line up. It's going to come out behind the intake as low as we can get it without hitting the wrinkle above the trans tunnel. We would have put it in the top of the tunnel but the trans is too tight for me to drill safely and it's not coming out just for that.

Call Ironworks - get a Mil-Spec bulkhead electrical connector.....

These cost way less and you can use a standard crimper instead of the expensive mil spec crimper.

https://paradiseracing.com/?post_type=product&s=Harness