Contiguous Block definitions

I would like to define a block of contiguous lines in the diff. In this block, no line may qualify as the beginning or end of a diff. This is an alternative to manual alignment, where I want to adjust alighment, but do not have matching lines between two windows, but instead want to align a line from file A between two lines in file B. For example, takes these thee inputs: (Base) a, b, c, d, e, f; (Theirs) 0, 1, 2, d, e, f; (Mine) 3, 1, 4, d, e, f. I want a merge result: 0, 1, 2, 3, 1, 4, d, e, f. (The automatic merge sees the line 1 in Theirs and Mine and matches theem, yielding <conflict>, 1, <conflict>, d, e, f). To do this using manual alignment it would have replace (Base) a, b, c with (Theirs) 0, 1, 2, then align (Mine) 3, 1, 4 BETWEEN (Base) c and d. Manual alignment requires alignment of one line to another, not a position between two lines. If I could define a contiguous block on (Theirs) lines 0, 1, 2 and another on (Mine) lines 3, 1, 4 then I would have an automatic merge result of <0, 1, 2><3, 1, 4>, d, e, f, which is what I want. The merge result doesn't have to be aware of the blocks, only the diff engine does.</conflict></conflict>