I've found older alternators don't tend to put out that much voltage. Between 13.8 and 14.2 is pretty normal for an old school unit such as a 1984 Defender.
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Something else to consider is the negative cable from the body to the battery. I assume all your other earth connections are to the body? The body is more or less isolated from the chassis and the current path is via that cable from the body to the neg terminal on the battery. Perhaps you should consider spending another $10 to complete the current path with new larger wire.
This is something young guns forget when fitting way too large sound systems to their cars.