I have a 600 cfm Holley Truck avenger carb on my Bronco and I am coming out to Super Celebration West. I am currently at 600 feet above sea level with 55 front and 57 rear jets. How far down should I go with the jet size? I believe the altitude there is 8000'

Jet Sizing For Altitude Changes
Step 1
Remove the jets and insert a smaller/larger set based on the vehicles operating altitude. When the operating altitude of the engine will be 2000 feet higher than your normal altitude, you'll need to insert a jet one or two sizes smaller than the size currently installed in the carb, which reduces the amount of fuel entering the engine to match the reduced level of oxygen in the air. If the altitude is dropped 2000 feet, you'll need to insert a jet that is one to two sizes larger than the current size, increasing the fuel flow to match the higher oxygen levels. The overall goal is to maintain the correct air/fuel mixture ratio for your specific engine. An incorrect ratio will show in decreased engine performance.

Might be worth having a couple options that you can swap out when you are out here to find the best fit. Based on your current stats-- it looks like you would want to move to 52/54 to start and see how that does...

Full disclosure-- I have been running EFI for years now since taking that same Truck Avenger off my truck.

Thanks, I'm going to order the smaller jets.

id order a size or 2 leaner and go from there. that will get you close.