I have a Holden,VX series 2 commodore, Automatic,V6 ecotec.I have a question should I get the 25mm manifold insulator or the 12mm as I would prefer to keep engine cover?Please advise

If you're adamant about keeping the engine cover then I would recommend sticking with just the 12mm manifold insulator. While you could run both the 12mm plenum spacer and 12mm manifold insulator and maintain your engine cover, it and or the bonnet liner would require to be modified