You will probably find that the overall diameter of the new 14 is about the same thickness as the old 16 gauge, the copper in the wire is the correct per size, but the insulation is thinner a lot of times. It is still fine.

If you want to do it right only solder, never just crimp. My opinion others may argue. If I solder in a splice where it is under the car/hood and water may get around it I coat with RTV before I shrink wrap it to protect the splice from possible corrosion.

Lastly while you are in there put a bolt-in bulkhead connector for the main harness power wires if it doesn't already have them. I like a fusible link at the alternator and the battery end of the main power wire. A shorted regulator can burn out the harness from the alternator putting out full power as quick as a dead short.

Another biggie. Check the insulators on the ammeter.