R6 is already announcing the 150.1.0.0/16 and 136.1.0.0/16 networks in the default configs to both BB1 and BB3. Make sure you drop those entries out of BGP or else the NAT exercise is pointless (although you will NAT all the same). Also, I had to use 2 seperate access-lists (same entries) for the inside source lists or else it would overwrite the previous ip nat interface config (so you can only have one ip nat statement that uses the same access-list).

Regards,
Nick