What's the difference?

Those are for the smog pump.....have nothing to do with EGR. The '74 360 4bbl intake will have slightly bigger intake ports than the '84 and as mentioned, the thermostat components are different. EGR came out in '72 in Ca. in the form off floor jets that metered exhaust gasses into the intake through a small oriface in a metal fitting threaded inside the manifold. '73 started an actual valve operated by vacuum. It was stock on all Ca. vehicles except the 340. Many 49 state vehicles came with a block-off plate and minimal smog for several years but eventually all states used this system. Even on EGR valved vehicles, many also have floor jets (or what looks like them anyway). Just look down the intake of any mid-70's and up intake and you should be able to see them. They don't seem to do much as I've removed them and replaced them with plugs and it doesn't seem to change the way the engine runs.