I was about to reply very much along the lines of Geoff. If you think that the FWF choices are confusing, you haven't scratched the surface yet. Aside from vertical vs. horizontal induction, vertical comes in flavors, too. I remember when people were modifying the engine mounts for the -H2AD (vertical at the rear of the sump). Then there's ignition choice, FI or not, inverted oil, remote cooler, not to even mention alternative engines.
But that's not helpful and I, too, understand Bob's (and others') concerns. Heck, I just fired off a question to Airflow Performance yesterday about something I'm not even sure is an issue; so I'm still learning with a flying RV. So instead of being defensive of the sport, let me make a suggestion. If you haven't already done so, get some eyes on your project. Not just the part you are working on, but also what you are planning. Not just a TC but anyone you can corner: other builders, A&Ps, anyone who can give you advice.
If you're building at home, find some way to contact local fliers. I was loosely involved with my local EAA chapter but, after I moved the project to the hangar, I suddenly discovered a network of owners, builders, mechanics, etc., that had always been there. Turns out that many of them flew for weekend breakfasts and I was able to hitch along with them; I got my long-overdue BFR on one of those junkets, not to mention some stick time and good advice from their experiences, even with 'spam-cans'. It was an IA from a few hangars down that advised me on my engine control hookups, and he later came back and helped me adjust idle and prop. He flies a twin with IO-540s and knows how the engines operate and guided me through a few installation pitfalls. He's not the only one, either.
What I'm saying is that even if you were to have a build-by-numbers FWF (and I hope the RV-12 guys are listening) it is still a good idea to have things looked over by people with experience. I helped another builder put a Walters in his Lancair Prop-jet, twice. I have no turbine experience, but I could spot areas where cables needed support and such; most of his technical support was from the guy who imported the engine. But when I was building my project, he returned the favor and found more than a few places I could improve; even though he had no reciprocating experience. Even inexperienced eyes can catch things you miss or propose solutions you hadn't thought of. My engine started up first time (well, after I remembered to turn the fuel selector from 'off'
) and the only problem was an oil line that needed to be torqued. I'm continued to get advice during the break-in and in the normal operation of my systems, and I expect I'll learn new stuff hours from now. But the aircraft is performing marvelously.
So, Bob (and others) open up your shop and get people in there. Homebuilding is a community effort; even the Wrights did not work in a vacuum. You'll get plenty of help and advice and sometimes you'll have to choose among options or make a compromise between some objectives. But you'll end up with a perfectly safe and exciting aircraft; these systems aren't that complicated or difficult. Buck up and press on.