I had a 70 Barracuda and had already moved the battery to the trunk so I had lots of room for duct line to rad support. I bought on of the fiberglass dual inlet jobs that encircle a 14 inch standard aftermarket air cleaner. Routed the 4 inch ducts to holes i cut in rad support on each side of the radiator.
The car was .25 seconds faster (low 13's) with NO other changes.
When done right they work.
The problem with the units for newer cars is that most newer cars already have a system that draws cold air.