Hi,
you dont need to configure a L2L for a pix which is configured in EZVPN client mode, you will need to configure a group on the vpn3k, just like you configure for vpn3000 sw clients.
if pix-501 outside ip address changes during the lifetime of ipsec connection to the vpn3000 concentrator, session would get dropped bcoz of keepalives.
hope it helps.
Thanks,
Afaq