My current & all my previous 68 & 9 Camaros have always been tucked acroos the top of the rear edge of wiper motor. But they all had factory or factory style wrap. No experience with aftetmarket wiring kits but if Painless supplies them with the later mid 70s style flexible conduit that likely explains why they need it routed below. The plastic conduit size/material won't allow the harness to sit down into the small gap between firewall and wiper motor is my guess. Current 68, not original wrap as I did replace/add some wiring in engine compartment diring equipment upgrades. But you get the idea.
A good quality cable throttle system like the Lokar is a good idea. Stay clear of using the GM style cables unless its all GM parts (Don't use OER plastic cable retainer, very brittle, poor quality & fit). I had one break despite very little operating force. Spend the $ buy the GM cable retainer. It works, fits as should & its made out of a different material and different color than whatever OER used. Just one more OER item that should never ne used.