I would just keep the specialty grains the same, and use 2-row for the 6 pounds of LME. Assuming a 70% efficiency, I'd just use 9 pounds of malt for the extract.
For hopping, did you do a full boil with the extract? You might have to adjust the hops if you didn't, if you want to duplicate the same IBUs.
I like amarillo hops, and centennial, but they are not really replacements for cascade hops. I'd bitter with the centennial and use the amarillo in place of the cascades at the end of the boil, if you can't find cascade hops.
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