I was chatting with a CHP officer today (no I wasn't getting a ticket), and asked him about the overall length law not being applied on primary interstate designated routes.

He assured me that no matter what freeway or road you were on, the 65' limit applied, and if you are found to be over 65', they would be hanging paper on you.

He added you could get an overlength / weight permit, but it would be just like a truck getting one, would be from point A to point B, and might require pilot cars etc. ( A PAIN).

I asked him point blank about the federal law taking priority over state law on a federal route. His reply was,

"If you got caught with a pound of cocaine in your bus, even though there is a federal law that was applicable and it was arguably on federal land, they would arrest you for violating the California Penal Code" (even if the federal law was more lax).

Hope this may help clarify the question!