[barabar]

Olddog,
You can do it. The flanges should be 1/4". 3/8" is to heavy and will cause alot of unnecessary weight and vibration on the headers. 3/8th is good for the header flange at the head. Take your headers appart at the slip joint and slip on your flanges leaving an 1/8" between the flange and the pipe.(believe me you will need that much room) Bolt every thing back on the car, bolt the flanges together. Tack weld the pipe to the flange accordingly. Now square things up and burn it in. Use some good welding blankets so you don't burn your car up. Another concern is the torque on your motor. With your headers solid to your motor and frame, the torque will cause problems. If you are running stock ford mounts drill 2 holes through each one and put 5/16 grade 8 bolts through them, you will eliminate torque over but retain vibration dampening.
Good luck.