I believe you will be in the same category as us last month going SEA-YVR-JFK to use the (wonderful) CX flight.

You do go through passport control and hand over a (blank) customs declaration, but via the "international transit" line which is supposed to be shorter.

However, if your arrival coincides with that of one or more of the many transpacific flights (or some transatlantic) that arrive mainly between 11AM and 3PM, you will wait a considerable amount of time even in the transit line, since many of the disembarking pax off the jumbos can't read the signs posted over the kiosks and mistakenly end up in the transit line. Rather than being sent back to the end of the (huge, crawling) regular lines, they will courteously process those pax, further delaying the transit folk. Then you are escorted back upstairs and into the departure area.

Now I don't know if same-carrier US-YVR-ANC transits are handled the same way but I expect they are, so bottom line, 45 min. sounds very dicey. YMMV. You might phone AS or the YVR airport authorities to see what gives.