If the lower intake manifold (the Y part) comes off, do new gaskets need to be used or can the existing ones be reused?

Do it once do it properly, use new gaskets. While your at it be sure to fit some platinum plugs in the rear bank, also check the leads n might as well inspect dizzy, rotor etc while at it. Also check that rocker gaskets are nit leaking as well. Otherwise you will be taking the gaskets off again.