Tubular exhaust manifolds
 

Can you guys help me out here? I need to know what year Ford started putting factory tubular exhaust manifolds on the 3.8 liter Mustangs and does either side have an O2 sensor fitting. Also, what are the dimensions like inside primary tube diameter and collector size? I've heard they will bolt up to 3.8 liter Thunderbird SC heads and I am thinking about trying it. I know there was an increase in power on the 3.8 liter Mustangs when Ford went to the tubular manifolds but I believe there were other changes as well. How much of that power do you guys think was due to the tubular manifolds? Thanks for any response's.
Jimmy