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NO alternator will "FIT" your 1948 8N. Brackets will have to be purchased or fabricated, and some folks add a little idler pulley to keep the fan belt away from the distributor cap. ELECTRICALLY, a good choice is a Delco 10SI, from a 1975 to 1985 Chevy pickup, sometimes sold under a rebuilder's industry standard # of 7127. The "7127" is followed by a "clock number". The alternator shown here is a 9:00 version, because with the mounting tab for the belt tightener strap at the top, the electrical connector is at 9:00. 
Although the alternator's case can be CAREFULLY partially separated, and "re-clocked", it is a good idea to verify the alternator you purchase has the connector pointing in a way that will work for you. You don't mention what coil you will be using. This diagram will work for an aftermarket 12-Volt coil, with a primary resistance of 3 Ohms, or a little more. 
If you ARE still using the OEM "6-Volt coil, OMIT the resistor shown, and instead leave the OEM ballast resistor in the circuit, and ADD a "converting resistor" of about 1.8 Ohms. The diode shown works WELL, or a "idiot light" with a #194 bulb can be used interchangeably. For more confusion on the coil/resistor issue, read the thread linked below, including my "rant".
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