Hello Xiao,
6704 is a regular ethernet linecard so using
switchport
switchport mode access | switchport mode trunk
is supported
SPA modules are WAN modules and you should use them when you need a L3 interface with advanced QoS features
see
http://www.cisco.com/en/US/docs/interfaces_modules/shared_port_adapters/configuration/6500series/76cfgeth.html#wp1155474
SPA ports are configured like router interface with vlan subinterfaces it would be a waste of resources to use them as L2 switch ports
Hope to help
Giuseppe