Pat, this is just my opinion, so take it for what ever it's worth. (You already know how I feel about Cobras).
Personally, I think your car would look even MORE period correct, if you left off the passenger side mirror.
Back then, (mid 60's) dual mirrors (for the most part) were rare.
I was going over a lot of old photographs of Cobras on line, and in some old magazines BKC (before kit cars), and most every photo (especially for the 427 model) had a single drivers mirror.
With no top, ya really don't need one for visibility anyway.
What ever you decide, it's going to look fantastic.
By the way, are you considering stripes?
Oh, and tell your Son, I like his Genesis, looks real nice sitting next to his Dads reptile.