The exhaust rockers are installed only once the head is bolted on to the rest of the engine, otherwise the rocker spindle would be in the way of the two bolts/studs.
In other news, I've spent a little bit of time on the smooth valve cover. Here's what it looked like when I bought it. And here's how it's looking now:
I used dry and wet sandpaper, some abrasive paste and about one fluid ounce of elbow grease. It's by no means flawless but I think I'll leave it pretty much like this. I like the dull shine it has, I think if it were as shiny as chrome it might look a bit cheesy.