Mine are 1/2" ID Schedule 40, which for what I was using them for, strong enough. No problem with them breaking. I picked Sched 40 because it is so much lighter and a little bit of flexibilty doesn't seem to matter. I didn't bother with the rope idea because I didn't think of it. I did have one of the eye bolts come loose on my last trip, so I will have to cut the end off and redo it, but rope might work better, so I may consider trying that.
Ed (the ski pulk website guy) uses marine stainless hardware, but ordinary stuff from Home Depot works just as well for anything less than big expedition style sledding from my experience.
What kind of line are you using? I'm thinking something like 4-6 mm climbing rope?
Tom, if you are redoing it the same way as the first time, I would suggest using a locking nut, or an adhesive on the threads to lock the nut. That should keep it from coming loose.