You will need an IS to post mount adapter, Shimano part number Y-817-37000. You can find one here, http://www.universalcycles.com/shopping/product_details.php?id=18669&category=31 , 7th one down, Item Number 18669-42793. And yes you'll need to run a 180mm rotor. Unfortunately adapting the direct mount IS caliper to a post mount fork adds 20mm to the stand off distance of the caliper. It's the only way to make it all fit and align correctly. You do have another option, kinda spendy, but it would let you keep your 160 up front. The M975 caliper, which yours likely is as 970s are Vbrakes, do come in a post mount front configuration. And will direct mount to post mount forks when used with a 160mm rotor. They are a bit on the spendy side at about $150 a copy new. But you should be able to find used calipers on ebay or craigslist cheaper I would think.
Your call of course. But if you do go with the adapter above, you will need a 180mm rotor.
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