recipe looks ok. might want to switch the LME to late and the DME to early.
Have you worked with 12lbs of extract before? it takes a lot of pot space.
can you boil it? can you cool it? high gravity solutions cool slower. if you're stove top you might need to do the LME as a separate pan, just disolved in a gallon or so to serilize it. cool it separatly and add it to the primary.