Thanks for the inputs on these EGR jets / floor jets. What threw me is that I didn't think EGR stuff appeared on 440's until 1973 - I got that info from the 440 Source website, which has a great page that shows pictures of all the 440 manifolds ever made, by casting date, and a short description. Here's the link: http://www.440source.com/intakes.htm

At any rate, I was able to remove them fairly easily, and I cleaned them up - I've attached a photo for those who have never seen these before, they're new to me.

I'm not sure what size plug these are (?), they seem to have some very coarse threads. I'll probably just put some JB Weld into their little holes and stick them back in. Since I'm going to block the heat crossovers with a valley pan, their shouldn't be much risk of a vacuum leak with these little buggers (but holler if that's a bad assumption).