Bluemonster,
The "shelf" that you describe seeing under the radiator cap opening sounds like the baffle that I referred to. Let me briefly explain why it's so important. When coolant enters the top tank of the radiator, it is deflected towards the center of the radiator core by the baffle. The baffle ends a bit to the right of the radiator cap opening (only visible if you were to remove the top tank of the radiator). Without the baffle in place, most of the coolant would enter the top tank and simply flow straight down the tubes nearest to the inlet, thereby bypassing a good part of the radiator core. The baffle forces the coolant to be distributed more evenly over the core.
So if you ever have that radiator worked on, don't let anyone tell you that you don't need that baffle. It's critical!!! Another thing - that's a hard to find radiator, so treat it with care.