78 K30 Carburetor Issue

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I have a 1978 K30 with a 454 that has never run like it should, so I decided it is tune-up time. Need to make it a more reliable ride. I knew the carb was a 600 double pumper, but have never paid that much attention to it. It is a model #4776, mechanical secondaries. I went to Holley's website to look up adjustment info and a found this as a description for my current carb.

"Intended for lightweight vehicles w/ manual trans or automatics with high stall & low gearing".

That is the exact opposite from what it is setting on: A 1 ton, 4 wheel drive, that gets used for hauling feed and hay and getting me to work when the snow is real bad...

Is a mechanical secondaries carb what I need for what I use it for?

What do type carb do I need to be looking for? CFM rating? Thanks for the help!