I have used plastic on LA engines with no issues, you just can't torque the screws too much. I put a washer under the head of the screw if it's an allen socket head, but the original screws come with a wide truss type head and can work, but a washer won't hurt there either. Use a good gasket or some good liquid gasket and you should be all set. Find the email address for Curtis Shipp here on the forums, as he has a pretty reasonable price for aluminum units. I have an email message in to him but have not heard back from him yet. He has his own machine shop business and is usually quite busy.
Type at you later,
Dan McEntee