I have a suggestion. When you do the headers is the time to replace your motor mounts with a set from Woodhouse. A couple reasons : The new setup will not have room for a flex joint and the new mounts will reduce the movement in the chassis without which you'll have some issues like cracks and bolts coming loose all the time, also, it's really easy to replace the RH mount with the exhaust out [ you can take it in and out through the exhaust tunnel in the frame real easy ], it takes a lot longer if you have to do it with everything in place. I would do this and add a 1/2 hour or so at the same time as the headers. You'll be glad you did.
Headers are actually pretty easy on a Gen III ... I'm doing a how-to for a Gen III. I hope to be able to post it in the next couple weeks.
Oh ya, I'd count on 10-12 hours, there's a couple things that may take a little extra time if you're being extra "particular" about what you're doing.