As said toss the washer (in your head see how much less the 5/8 will flow as opposed to your 1.5/1.75 rad nipples for instance). Use a good (Milodon?) high flow stat 180 or 185 to get the eng out of the cold/fast wear zone as quickly as possible. If the washer ain't it (& I'd be surprised if it ain't) then as said the fans are inadequate or are blocking airflow. I'm assuming the rad was flowed or at least checked out a bit (running a garden hose in it is a good backyard test for flow). Holler how it turns out.