According to your description, ISP is running "bridged 1483" encapsulation. Please confirm with them, if that is the case, configure:
int atm0.1 point-to-point
pvc 8/35 <-- or whatever else
bridge-group 1
int bvi 1
ip address 1.2.3.4 255.255.255.240 <-- As given by ISP
ip nat inside
int vlan1
ip address Y.Y.Y.1 255.255.255.0
ip nat outside
bridge irb
bridge 1 route ip
bridge 1 protocol ieee
ip route 0.0.0.0 0.0.0.0 x.x.x.x <--- default GW as per ISP
ip nat inside source list 1 interface atm0.1 overload
access-list 1
permit ip Y.Y.Y.0 0.0.0.255
Hope this helps, please rate post if it does!