If you are running all the newer holly lsx swap parts, you shouldn't have to raise the tunnel. I just dropped that same setup in a 69 (T56, not a magnum). In reading their paperwork it should work without cutting the tunnel with the magnum as well.
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69 RS LS3/T56
68 firebird vert LS1/4L60E
67 RS LS2/T56
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