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."
Something to keep in mind. 307 parts are not that plentiful and they generally cost more than 305/350 parts when you can find. If you just want a free engine to run until it dies, I say go for it. If you want to make some mods, like cams and things, I would reconsider. Look through Summit and Jegs catalogs for 307 parts. Hardly any that I remember. Just sayin...