New carb for 1965 273 4-bbl hi-po package

My original carb was toast as well. I put on a 1405 and it has been trouble free for 10+ years now. You won't notice much difference in the 500 vs the 600. I would leave it as is. You can always tune your 600 to make it work better. Metering rods and springs do a lot for those carbs.