I'm a big fan of overkill. The way I look at it, if you take care of it the rope will last for many, many years. I had the company put thimbles in the ends of mine, so I didn't have to tie knots. They also put a 6' anti chafe sleeve over one end for me and used heat shrink tube to hold it in place. I got a pulley where the sides rotate so I can put it around the rope with the thimbles on the ends. The pulley is rated for around 10K pounds I think and it is stainless steel - no corrosion. I've had the rope for probably 10 years and have used it for many things. I think I paid around $3/foot for the whole thing and it is well worth it.
I don't know if I'd go smaller than 3/8" of that amsteel rope for your application, and wouldn't feel bad going 1/2".
I believe I found the pulley on an arborist supply site. I use it to tie off to a tree so I can direct the way the tree falls if I can't get a straight pull with the tractor. 65 hp, 4wd tractor can put a lot of tension on it when you have the FEL bucket full of dirt and in low gear!!
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