According to the Haynes manual,
60 NM, or 44 Ft lbs, for 1st step, then additional 90 degrees for second step, then tighten cylinder head to timing cover bolts. cylinder head bolt tightening sequence, I hope you have. I will try to explain this.
Looking at motor from front, you have left side and right side, Seven Cylinder head bolts on each side. On left side starting from front, the sequence numbers will be 13, 7, 5, 2, 4, 10, 12 the right side will be from the front, 11, 9, 3, 1, 6, 8, 14. Hopefully someone can post a diagram of this for you quickly.
88 XJ6 VDP - Sold
87 XJSC V12