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^^ You can do this, but your OEM shocks will be compressed past specification. That will affect both durability and function.
+1
Besides that, if you're past 70k on your original shocks, it's a waste of time and money to install springs without replacing the shocks as they won't do their job (or do it very well) of controlling them new fangled springs.
If they are still good, you're going to shorten whatever life you might have in them with stiffer springs.
If it's a budget thing, save up for new shocks to go with the springs. Even some OEM equivalents are better than your tired old things.