Thank you all for the great responses.
Sway, the math really helped, thanks for taking the time to write all of that out.
I think I may be okay, the load is actually less than 1000lbs, more like 600lbs, I was being conservative and wanted it to use 1000lbs for a safety margin. Now I see that the working load on the equipment already has a safety margin built into it at 5-to-1 or even 10-to-1 to estimated break, I just hadn't read the fine print and didn't know they were already being conservative. The load won't be lifted more than about 60 degrees from vertical, and the rings I am looking at now are forged steel rated over 3000lbs WLL and are 10-to-1, rated over 30000lbs to break, that should more than do it (famous last words).
Edit - follow-up question, is there a resources somewhere that says what the estimated holding power of various knots and splices, etc, are for lines ? I would assume that the goal in using most knots would be to have the rope break before the knot, but I don't like assuming things, so it would be nice to find out what chances there are of various knots slipping.
Edit #2 - I forgot to say why rings since that might come up, the reason is that I need to attach the load to the rings in multiple places, not with a single line, so I need some surface area to work with and the rings would let me attach the load with a number of lines. I think the shackles might be too cramped though you guys have totally convinced me that is the better route to go if possible.