BD S10 on 20s firewall question

This weekend I am going to go ahead and tub my firewall for the 20s since the motor is still out and the new one will be going in soon. I have not finished bagging it yet. I need to know how far into the fiewall the tub will be, how wide it will need to be and how high to go on the firewall. It's for a 245/35/20.