Joe I've got one of two different ways you are going to attach your main harness to your firewall.
The first is a ring style clamp that will hold the harness. I've the large style ones that will hold the entire harness. Also there are holes in the "lip" that comes off the firewall. This method is the best looking.
Second you can use this bar style push pin thing. you can wrap electric tape around the loom and bar. Then use the holes to hold it up. It's not as good looking.
Either way is fine, but I suggest using the clamp style. This can be done after the engine is in, but it's easier with it out. I'll swing them out tomorrow afterwork if you plan on doing it tomorrow night or this weekend.
Here are examples.