Do you have to rebuild a 350 engine replace camshaft?

No, you do not have to rebuild a whole 350 engine to replace the camshaft. Replacing a camshaft is a significant job, requiring engine disassembly to access the cam, but it's a common repair that doesn't necessitate a full rebuild. You'll need to remove the valve covers, timing chain or belt, and likely the oil pump, but the engine block itself doesn't need to be disassembled for this procedure.