1. If I buy a Shockwave with an 18.5" barrel with the birds head grip, is it legal to replace the birds head grip with a regular butt stock?
Yes.
18 inch barrel is a regular long gun (title 1) weapon.
2. If I buy a 590 with a regular butt stock, is it legal to install a birds head grip instead of the regular stock?
I'm assuming you meant a 590 shotgun with an 18 inch (or longer) barrel. If so, the answer again would be YES. You can use either a stock or a grip.