I think I located mine by hanging the eng/trans in there with the mounts attached(eng hanging from the hoist and a floor jack under the trans). I moved it as far back as I could get it and still have room for the kick down rod to go between the bell housing and body. I think there's maybe an inch clearance between the back of the valve covers and the firewall.
I didn't cut out the crossmember that runs under the bell housing. I probably will have to cut it if I ever have to remove the tranny without pulling the motor. I think I've got enough clearance to pull the tranny pan and change the filter without notching that crossmember, though.