I´ve fitted a 770 middle ring to a 760 crankset with no problems. Actually, I´ve done it on two cranksets already. Mine has a 770 carbon/steel middle ring for already a year.
You´ll have to shave a little bit of aluminum off the spider (see pic), and put a chainring spacer to center the ring properly.
It takes less than half an hour if you have a dremel.
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