I am wondering if I should put the fuel selector in the plans position or not?
Only you can decide if you want to deviate from the plans or not. Nat would tell you to put the plans valve in the plans location. I think I'll like my Andair valve better up front, but most Cozy's are built with the valve in the plans location and it is OK.
Can I just leave it out and worry about it at a later date or will this come to bite me later?
Build it like the plans show until you come up with a new plan. You can't skip portions of the build if you don't have a really good, thought out, new plan. So yeah, for now, just build it like the plans show. It'll teach you to bend aluminum, use rivets and nutplates. My bracket is in there too... even though my valve is up front.
If you KNOW that you won't install a valve in the seatback... then you can skip the bracket.
Will the SDS EFI fuel selector fit in the seat back? Is there a diagram for the plumbing of the SDS EFI system? Will I be able to retrofit this system if I leave it out at this point? Again thanks for all of the feedback and help.
The SDS EFI fuel selector is a 6 port valve which switches both the tank your fuel comes from, and where the fuel is returned to. Go to the Andair website and check out the FS2020 duplex valve. This is what you'll be using (or something like it).
Will it fit the seatback? Sure, one way or another you can fit it there. It won't fit the plans bracket, but you can retrofit it later.
I haven't seen any photos posted of one of these duplex valves installed in an EZ or a Cozy. There are likely some installations, but they haven't been shared with the community. Marc works on a lot of these planes, perhaps he can share photos of a duplex valve installed in the center console or in the seat back?