The baffle( windage tray) will be different. The 3.0 will sandwich between the pan and the motor. The 3.4 will be inside the pan. I opted to modify my 3.4 baffle to fit inside the 3.0 pan which was just doing some minor cutting and drilling.
The rule of thumb is that whatever pan fit before your swap will still fit since like Oops said, the block are physically the same on the outside. If you also planning to a straight axle, you'll want the 2wd t100 pan.
Link in my sig to my 3.4 swap saga. I covered the oil pan in there if you are wanting to take a peek.
