With all due respect to your mechanic friend - he's dead wrong. I would encourage you to buy high quality parts and follow installation instructions carefully and you can have a beautiful engine without leaks.

Most every car I every owned has been customized to my liking which includes after market bits and pieces. Again, the key is quality components. The $39.95 kit from your local Pep Boys will be more likely to leak than name brand parts from Summit. I've used chrome GM valve covers on several cars and never had a problem. They run about $50. Include good breathers at about $10 each, FelPro gaskets and chrome bolts and for less than a hundred bucks you've added a touch of class.

The timing chain cover is a bit trickier. Obviously, water pump must be removed as well as the harmonic damner. It's hard to get a good seal sometimes at the pan without removing it.

My avatar shows a SBC with what you're looking for. Some chrome, billet pulleys and after market intake (Holley Z-28). I've replicated this look on several engines without leaks or cooling issues.

Have fun, take your time and let us see the results!
Glenn