Back trolling that stretch of the Willy can be productive and using a jet is the right choice of diver. 50/50 might be a tad deep. All my successful hooks up occured between 12' & 25'. Often the notion that Chinook run deep in the channel isn't the case on the big C or the Willy. 12' to 25' (as mentioned) is the golden zone.
Using a flasher would be up to you, based on water conditions, color and clarity. Being the W... I'd suggest nickel & green.