I also have the old-style castings and mine only have four bushings installed (two on each side). My bushings are inserted into the top of the casting holes at the locations in the image below ie., no bushing on the far bottom (my boat came this way when I bought it - not sure if I'm supposed to have the third bushing but two seems to fine). Also, for what it's worth my old-style casting bushings were inserted into the holes from from the top whereas the instruction manual shows them being inserted from the bottom with the new castings.
When I replaced my bushings they punched out easily with a screw driver. I don't know why you would have to open up the holes to install new ones - just lube 'em up a bit and tap them in.
