you are correct. you can do the mirroring using SRDF technology. although iam not sure abt the distance limits for host based mirroring.But SRDf is a good option for you to make R2 mirror on EMC box B for R1 volume on EMC boxA using SRDF.
you can setup VCS also using SRDF.. there is a VCS agent for EMC SRDF which handles SRDF.
you can get more iinformation on SRDF on EMC forums/sites.
Using Volume Manager mirroring over this sort of distance is not uncommon nowadays. New features in VxVM 5.0 allow for site based allocation and site consistency. However, Symantec only certify VxVM mirroring up to a distance of 80km.
You can also use VVR to provide synchronous/asynchronous IP based volume replication. This has no distance limitations.
What type of cluster are you setting up? Will you be using GCO? GCO is not supported with VxVM mirroring. You would need to use some sort of replication tool such as VVR, SRDF, TrueCopy.