From what I can tell comparing the two is that the Jim Carter harness specifically mentions turn signal wiring, and they offer both GMC and Chevy harnesses, while Classic doesn't offer different harnesses for GMC and Chevy. Maybe the extra turn signal wiring is worth the extra money. Disclaimer: I have no experience with either one, having repaired and upgraded my original wiring harness myself.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.