The answer depends a lot on how much idling/low speed operation you will have during peak loads. Output increases with speed until the point of maximum is reached. This is around 1500-1900 engine RPM in most cars.

The reason most manufacturers had increased alternator output in the 1980s was that the switching transistors used in early electronic ignition systems had significant enough loss (on-state resistance) that system voltage became a factor. Larger alternators were used to keep ample voltage at idle. The other popular accessory of the time is the resistive type rear window defroster, many taking 25 - 40 amps at the same time as defroster/heater fans and wipers were often cranked to high.

The capacity rating of your battery is irrelevant. What counts to the charging system is the amount of energy you consumed during engine starts. Like using a gallon of gas - it doesn't matter whether the source was a two gallon can or a hundred gallon tank - it's still one gallon that you have to return to make it full again.

300 watts is 20 amps (at 15 volts). You will have AMPLE reserve for engine needs + battery charging. You don't factor short-use loads like brake or glove-box lights, or interior lights that are not on while you're driving. You also do not need to add trickery like adjustable voltage regulation because that will only affect higher speed operation if your loads exceed the alternator output and reduce the voltage at lower speed. In other words, if the alternator's output is maxed out by the electrical load for a given engine speed, the voltage regulator is calling for 100% until the speed increases and duty can be reduced. It does not matter whether the regulator is adjusted for 13.6 volt or 15.2 volts. The higher voltage setting only makes the alternator work harder by waiting longer to cut back from high charge rates.

About the only thing you really need to do is ensure that the belt is not worn too narrow and remains tight so that you can get the necessary mechanical energy into the alternator shaft (merely spinning it does not allow charging).

The other factor: if you were to place a Bosch 70, 90 or 100 amp alternator next to your 55A, you'd see that the pivot bolt casting is much longer on the larger unit AND the adjuster mount will be on the side away from the block.

One favour... if you're driving behind me, please kill those grill and fog lights that do more to blind the drivers in front of you than they do to help you see the road.