The easy way to put a big load on a cable is to stretch it between a
couple of solid supports and hang a weight in the middle. You can work
out the vectors with nothing more than some graph paper, a pencil, a
ruler, and a protractor--and you'll be surprised at low little weight
it'll take. You do need to know the modulus (i.e., ratio of pull to
elongation) of the cable you're using, and the supports have to be
_really_ solid.
Alternatively, if you have access to a 700-lb spring scale or load cell,
just put that at one end of the cable and add weight to the center (or
just step on the bloody thing!) until you get the desired strain.
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