I refer you to the following command reference on 7.0 http://tinyurl.com/22uhjdv
command for enabling transparent mode is since version 7.0
pix 501 does not support 7.x and later code http://tinyurl.com/2782lmk
In short, firewall mode transparent is not supported on pix 501.
Sorry that it does not help your cause, but hopefully clarifies the pix501 capability.
Maybe you should instead make the modem transparent, or if possible make the pix a pppoe client to your provider http://tinyurl.com/2cumtns
Regards,