The alternator will only put our what it is allowed to put out depending on the current sent to its field circuit (Orange wire with light blue stripe). Cleary 18.4 volts from the alternator is too high. To determine if the problem is cause by the regulator, the alternator, or some other cause, I would disconnect the Field Circuit at the alternator (do not let it touch bare. If, after the Field Circuit is disconnected at the alternator FC terminal, you are still getting 18.4 volts out of the alternator I would agree the alternator has a problem that is causing the field circuit to be getting current from somewhere it ought not be getting it. If the voltage is below 14.8 volts or so (likely 12 - 12.5v), the problem is not the alternator. With the 4 wire plug disconnected from the VR, check for voltage at the Field Terminal wire for the alternator (Orange with Blue Stripe). It should have no voltage. If there is voltage present you have a short in your wiring that is letting current feed into the Field Circuit even when the VR harness is unplugged. Once the VR 4 wire plug is plugged back into the VR you should then be getting some voltage at the Field Circuit terminal, as it is that current that tells the alternator to produce more or less output - depending on battery voltage.
My gut tells me your issue is most likely a VR that, despite being new, is allowing the alternator to get excessive current sent to the alternator's Field Circuit terminal. 18.4 volts is definitely excessive, and I am surprised you have not have more problems than a hissing battery. The battery hissing is not good, the battery is cooking off due to excessive voltage and heat, The plates inside the battery could end up becoming deformed which could cause plates to begin to contact each other in places they ought be contacting.
As for a replacement VR, if indeed it is sending excessive current to the Field Circuit, I am not sure where you are buying yours from, much less the brand. I personally like NOS Motorcraft, whether an older electromechanical or newer electronic design. If you must sue an aftermarket brand, I would stick with a high quality brand, such as Blue Streak.
Once you do resolve your charging system issue I hope you post what eventually did correct it.
