I think I felt the same as you when I got my firewall forward kit and engine late last summer. The rest of the build was pretty straight forward, but the big mess of systems on the front side of the firewall was by far the most intimidating part of the whole project. Now that it's "done", I see that it was just another progressive learning experience. I figured it out as I went along, learning lots, making more than enough mistakes.
As others have mentioned, figure out and install as much of the firewall pass through and attachments as you can before installing the engine mount. I did, however, end up with extra holes in the firewall that I had to patch/re-purpose. This was mostly because the plan locations for the control cables didn't work for my installation (IO-320, C/S prop). Almost all of your firewall forward systems have something attached to or passed through the firewall - electrical, fuel, oil, cabin heat(exhaust), sensors, controls, carb heat/alternate air (intake) and engine. This mean that the more you know about where those systems sit in your engine compartment, the better results you'll have in the firewall planning. The single best resource I found for this was the 360 view of the firewall forward of the RV-14 that is on the Vans site. Yes, the engine is different, but it gives a pretty good idea on the interactions.
Timing on the riveting of the firewall recess is worth paying attention to. There are a lot of the rivets that get used for platenuts and holes that are drilled out for bolts. Once all of those are figured out, along with any cut outs, rivet this in place - before you hang the engine. Would also be a good idea to seal that area before the engine gets in the way - it's a difficult spot to reach later. Of course when it comes to riveting things, wait as long as you possibly can before riveting in that top forward fuselage skin. The access that you have to the aft side of the firewall is crucial for a very long time. Only rivet that on when you have nothing left to do. It can be clecoed into place for fitting your cowling.
One specific gotcha that I'll pass along - the firewall flange rivets. For the most part, this will be done when you figure out your cowling attachment. There are a handful of rivets on the underside that do not attach your cowling - in the center in the exhaust area of the cowling. There are a few of these rivets that are near impossible to set once your exhaust system is installed. There are a couple that you drill out for the air outlet support bracket and fuel overflow, but riveting the others before you hang the engine mount will be much easier.
Most of the firewall forward is simple enough if you think of it one system at a time. The baffles are time consuming and somewhat tricky, particularly the oil cooler. Fitting the cowling is both time consuming and difficult. The cowling trims are hard to figure out and even harder to fix after the fact. Take the time to get it right and you may be able to not regret how it turns out.