1: cut the old center pin out of the existing box.
2: disassemble the 705, discard the lid, and trim the box into a new lid. Cut off box sides and rear.
3: use new lid with triangular center pin, together with new coupler, placed in old box on car, to find location for drilling and tapping for new mounting screw.
4: remove 705 coupler and new lid, drill and tap car box, then reassemble new coupler in old box with new lid, spring, and screw.
See photos for details.
This is a Rail Line boxcar, this technique should work for blackstone cars as well. The centering works, and the coupler head is nice and close to the striker plate like it should be.