Help needed for wiring harness on LS3 swap

[MadMaxx]

So after almost selling the car 3 times in 10 years due to a divorce, money sucking lawyers and losing interest I think I may actually live to see it get done- 69 Camaro.

So I bought a LS3 525 hp crate motor almost a year ago from a GM dealership. It came with the ECM, wiring harness and gas pedal. I'm really close to needing the harness but am very concerned as to what the hell I need. I know they make harnesses for 69 Camaro's with LS motors. I was told the ECM, gas pedal and harness that came with the motor is Junk and I would need to get new ones per Spear-tech ( why?) I want this done right but just want to understand why and how without spending tons of extra money on stuff I don't need. The guy I spoke with seemed like he wasn't interested in my business or in helping. I don't understand how the LS3 communicates with my factory gauges inside the car. I have new factory tach, new factory gauges in my floor council. I'm running a tremic 5 speed. I have electric hide away headlight motors, after market A/C. I have no clue who to listen to or who will take the time to help. The car is brand new everywhere and has cost lots of extra $$$ because new stuff came out over the past 10 years and well you know how it goes.
Any help, suggestions are greatly appreciated.

THANKYOU.