You don't need the PARAM.SFO for the US version, just change the PARAM.SFO to 3.55 and let Multiman do it's magic.
Finally, another brief guide from ianrobbie, to quote:
Downloaded both files above (thanks for the fix, by the way michaelscz!)
Replaced the original PARAM.SFO with the one downloaded from above (as I said before, it's just and EU PARAM.SFO)
Let Multiman change the PARAM.SFO to 3.55 (Don't know if you can do it yourself)
Once back to XMB, installed the above update .pkg
Loaded the game!
I only got as far as the "Installing" screen before I had to switch it off. I would probably recommend transferring it to internal. It might speed up the installation. It started at 0.01%!
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