Well, let us see how it goes with iron when you get to it. It was quite a learning curve for me. I haven't cast iron yet this year as conditions are not yet right -- first too wet in the spring, now too dry -- the whole state is presently under an outdoor burning ban.
It's funny, I started out this project thinking I was going to do most of the major pieces by fabrication in steel, but have ended up casting all but the bed.
I might just switch back to steel in doing the headstock. It's not a sliding component, except for initially boring the bearing recesses, and I'm running low on zamak. I don't favor aluminum here -- steel will give me more rigidity and mass in the same thicknesses, and I definitely want that in the headstock. Cast Iron is still not do-able for the above fire reasons. So steel might be the best choice.
Zamak has worked well for castings, but is difficult to work with -- especially with hand tools. Filing practically doesn't work -- it is both hard and slippery, and a file tends to skid across the surface, and won't bite in. It clogs grinding wheels, unlike iron or steel.
I would NOT recommend zamak castings to anyone building a first lathe like the Gingery. Or anyone who didn't own a milling machine to work it. I think aluminum is ideal for the Gingery lathe (except fof the half nut, which should be zinc alloy for wear resistance and bearing qualities). Dave Gingery suggested either alloy would work, but I think zinc alloy would be torture for a beginner with only a set of files and a homemade scraper. I doubt the lathe would be finished after a difficult start.