Need help from any electrical experts

Guys -
I'm in the process of re-wiring the main charging circuit of my 66 Dart GT as outlined in the madelectrical "Bypass Ammeter and Bulkhead" project.

The new wires added are 10 AWG, whereas the existing wires are 12 AWG. Will I compromise the existing wires connecting them to 10AWG? Will I improve anything? Does it matter? It seems to me that unless I replace the entire wire, the circuit will only be as strong as its weakest link, so is there any reason at all to use 10AWG when part of the existing wires will be 12 AWG?

Thanks for any help with this!

-Andy