If production ever increased, I would use a Passon 5 speed Hemi box for your application. Your 600 ft-lbs engine torque is only relevant if you are putting all of it to the ground. If you are racing on slicks it might be more critical, but I am guessing that this is a street car since you want a syncro box. I'm still using the same hemi box in my 'Cuda since the mid 1980's and my car has run high 9's. The new motor will push it into the 8's and I'm planning on using the same trans, just different clutch setup.