If i use the Th350 crossmember from spohn do i still need to buy the b&m conversion kit?

No.

Providing you're also using the Spohn torque arm that goes with the crossmember.

I think Spohn makes a X-member that is just to retro fit the T-350 in there. If thats the one your talking about than you will still need the B+M bracket to mount the torque arm to the tail housing.

If your talking about the one that comes with the torque arm and mounts it on the x-member than you will not need it. I suggest buying this one