The O'connor bushings are the only way to fly. They create a seamless install for ANY 400EX rear shock.
The bushings create the proper spacing and bolt cavity for the upper shock mount, but you do need to either lathe the stock top blaster bolt down to 10mm or replace with a 10mm bolt.
The bottom of the 400EX rear shock gets drilled out to 12mm to take the stock rear shock blaster bottom mount bolt. No need for a spacer or shim on the bottom, the shock is designed to "float" in that bottom mount to the swing arm, works just fine.
OR, you can do, as Freek said and did, fit a few washers in there to secure the fit a bit more, but again it is not necessarily needed.
AND, NO, running the 400EX rear shock does not stress the rear of the blaster/frame anymore than the stock shock did, just provides a much better, stronger, more adjustable rear suspension setup, by all means.
Hope this helps...