Take the created box8 apart to define the macros |@|"col" and |@|"row" containing the width and height of the bounding box of the entries in the row/column. Then set |@|"col" and |@|"row" to set X and Y of the upper left corner of the bounding box of the entry at "row" and "col", "relative to the center of the top left corner entry", , |"1,1"|:
( @@measureit@MATRIX MEASURE 6=8 @=@ @ @ii=@<@ 4=@=4 @4 @=@ 2=4 @=@ @ @ii=@<@ @= @@ne @ @@ne @@=ne @=Wcol@1@=-.5@ @ @Wcol@@ @@ @<@ @ne @ @=ne @=Hrow@1@=.5@ @ @-Hrow@@ @-@ @<@ @ne @ )
: Allow non-default spacing between particular rows/columns as well as different justification of the entire matrix...