[HO455]

First off I apologize somehow I got the idea your truck was a 67 thus the tangent off on the other alternator styles. The wires to be upsized would be the alternator output wire (red 12 guage on 1/4-20 bolt on the back of the alternator. And the battery charging wire (red 12 guage wire at junction block next to battery). I recommend you look at the first link for a good explanation of what wire is which and what size to go to and why. The quick answer is change the battery charging wire to a 10 guage and the alternator output wire to an 8 guage if your going for the 94 plus amperage alternator.
If you look at their Chevy diagram and compare it to a 67-72 C10 wiring diagram (2nd link) it is pretty easy to understand what they are talking about as it applies to our trucks. Trucks went away from the horn relay wiring they talk about on pre71 years earlier than cars did, but they didn't go to charging the battery through the starter in 71 like the cars did.

http://www.madelectrical.com/electri...evymain2.shtml

http://outintheshop.com/faq/wiring/wiring.htm