Can this ATMega328-PU ATMEL microchip be used instead of MCP3008
Yes you can, but your tutorial breaks down because you are not following it. 

The ATMEGA is actually a microcontroller. It requires more attention to setup the hardware as well it requires programming. If you are a complete bigger and want to follow the tutorial, then this might not be the approach you want to take. That's not to say you can't do it, it's just slightly more complicated to get working.

The MCP3008 is an external 10bit ADC. It is not as complicated to get working than a microcontroller.

If you can find a 10bit external ADC, you should be able to follow your tutorial pretty well.