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Chevy 350 has loss of power under load.?

I have a 70s 350 bored .030 an it was rebuilt 3 years ago. It has 52,000 miles on a rebuild. I have replaced the fuel pump, New hei distributor an cap rotor coil. All new spark plug wires an spark plugs autolites ap26 an installed a new edelbrock 1404 500cfm carb. It has always had a 500cfm carb an ran fine before until now. It has absolutely no power under a load. I repaced the intake gaskets an checked all pushrods an lifters for wear an everything looks absolutely perfect nothing excessively work. The heads are the junk 333882 heads which to me are a trash head for cracking. I even replaced the pcv valve an checked all vacuum lines. I tried using higher octane fuel 93 octane an used fuel additives an nothing has made it change under a load. The timing is perfect an I have adjusted it every way possible an no matter where it's at it's got no power. Im not sure what else to do. I'm thinking of building another 350 since this one has lost power under a load. The only thing I noticed was major carbon buildup on the intake valves when I had the intake off. I cleaned some of it off. Any help is much appaerciated. Its not using oil either. I just don't know what else to do. Its only a daily driver

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