Everyone is different and everyone reacts differently to a physical challenge like that. It's ultimately going to have to be your call.
Do they have SAG support to pick you up if you try the 65 and it's too much? Would you respond better, motivation-wise, to challenging yourself and perhaps not making it, running up against your limits and trying to force yourself over them - or doing a ride you know you can do and attacking it with a will? You'll have to decide.
Looking at the climb you're pointing out, it looks like it stays under 10% for all but one little bit at the end, which isn't bad. Maybe comparable-ish to Kings Mountain?
Im not sure if they have a SAG support. BUT a friend of mine who isnt doing the ride will be in Napa. Im sure If Im laying on the road looking at a white light she can come grab me in her car. And as long as its just one little bit and not a sustained 10% grade I think I can do it.
I did From Chrissy fields down into Sausalito> Marin >Mill Valley> Corte Madera >Larkspur> Kentfield> San Anselmo> Fairfax and back (only 37 miles :|).
That hill on Alexander was the last little bit I had to do at the end of the ride and I remember what that felt like. It was my first ride ever. And although I did it I was paying for it big time for awhile. If I dont have to do Alexander type hill on Napa I may just go ahead and do the 65er.