[dmjlambert]

I understand the point of the fusible link is to burn up first instead of burning up your main truck wiring, and when it does burn the insulation on it melts and smokes but doesn't burst into flames.

If you go changing the wire sizes, you may need to be aware the stock battery gauge is an ammeter and it depends on the battery to alternator junction wire being 12 gauge. That length of wire across top of the radiator support serves as a shunt. Some people don't mind that because they don't have a working ammeter or they have replaced it with a volt meter. Or may not have a gauge type instrument cluster.

Probably the safest and easiest way is to use fusible link wire. It should be 2 wire sizes smaller than the smallest non-fuse-protected wire, which is 4 wire numbers higher. On a stock truck the non-fuse-protected wires are all 12 gauge, and are protected by a 16 gauge fusible link wire about 6 to 9 inches long. If parts of your wiring are something bigger, such as 8 gauge, you will still need a 16 gauge fusible link because some of the wiring in the cab that is not fuse-protected is still 12 gauge.

Your truck sounds very much like the way I found my truck when I bought it. I spliced a couple feet of 12 gauge red wire onto the red wire that was coming through the wire holder that goes across the top of the radiator, to make it long enough to reach the junction block by the battery (like it once did when the truck came from the factory). I used a solder connection with heat shrink tubing to splice it.