Exactly . If you look at a modern car you will see that all holes and gaps in the core support are filled with something (foam tape, rubber, plastic plugs). This makes it so all the air is forced through the radiator (one of the reasons many cars can get away with those thin, cheese radiators).
Sometimes you will see people that have added an aftermarket radiator to their a-body (or any car for that matter) and suddenly their cooling goes in the toilet One reason I have seen is improper fit, Im talking that 1" gap all the way around the radiator that allows air to go around and not through (especially bad if you ditch your shroud at the same time).
Fill those gaps, plug those holes and your cooling efficiency will go up.