Those four bolts are not easy to remove. I sort of remember modifying a box end wrench to fit on them. Grinding a portion of the outer edge so it would fit on those bolts.

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1983 Mercedes Benz 300D Turbo (Daily Driver)
1979 Mercedes Benz 300CD (Needs Lotsa Work)
1963 Chevrolet C10 Stepside Shortbed (Not Stock)
1999 Chevrolet Silverado Z-71 (Fun to Drive)
2007 Nissan 350Z (Wifes Car - Lucky!)