Hi, can you confirm if doing redistribution BGP to OSPF on the 6509 switch (in red) would be the typical way to allow routing of my intervlan and then allow me to route traffic in/out via the AS 1803 (ISP)?
Please see attached diagram for detailed explanation.
until both C6509 have complete knowledge of internat network you should be able to route to outside world using default route that the C3750 stack can inject in the OSPF domain.
For more safety I would provide a direct L3 link between the two C6500 so that should an SVI be shutted down on C6500_1 the traffic destined to that subnet is sent to C6500_2 and not to the C3750 stack.
If you don't use passive-interface on client vlans you have already multiple parallel links between the two C6500.
The C3750 stack can advertise internal network subnets as described in my first post: using network command under router bgp (preferred) or by using redistribute ospf (to be used only if the number of subnets makes not feasible the network command approach).
Hope to help