All Classified forums require a Site Supporter subscription. It is only $25 per year and you can buy and sell whatever you like on the forum. To become a Site Supporter, go to Your Profile and select "Paid Subscriptions."
Maybe I am confused, but you only have to drill non 400 heads to go on a 400 Block. To put 400 heads on a non 400 block you just need to use a 400 head gasket.