Update & question on erratic cooling

Continuing from my previous thread. Today I stuffed a garden hose into the 2 1/2" inlet on the radiator. Water passes across the top of the core and out the bottom pipe really well. I now believe the core isn't stopped up. However if I push water up through the water pump bypass hose into the top tank, the top tank fills almost immediately and very little water comes out the bottom. How big is the passage from the top tank down onto the top of the core? If water won't leave the tank down to the core, how can the bypass hose be effective? I really don't care to unbolt the hood springs from the top tank bracket, unbolt all the bolts securing the tank to the core just to have a look at the bottom side of the tank. TIA