Thank you for your investigation n2fordmuscle. I didn't think that this would be so difficult to find. It' more easy for the Mustang. I don't despair, I continue on my side.
The long tube headers I used on the 78 Thunderbird would likely work on yours. They were listed for a 72-76 Ford Torino with the 351/400M. Since these two engines use the same heads, they will work, and the front end layout should be the same, with yours having slightly more room, thanks to the longer wheel base. Thing is, the headers were $700, and the ceramic coating another $350. And they blasted things kept hitting ground! I'm going to check around here, as it should not be that hard to find manifolds for the 400M engine at all! There were LOTS of them made.
There are some very wise men with some very good advice on this forum. Bullit69, stick only to headers that are made for your vehicle body type only. You will create so much more grief for yourself if you try to adapt block huggers or headers that were designed for similar but not the same models. Have you tried talking to Heddman or Hooker headers, if only for some professional advice? I am guessing that there would be someone there that will be able to give you the exact info you are looking for. Good luck with this, cheers Mike.
Do either of these look familiar? http://www.ebay.com/itm/Ford-OEM-Ex...r_Truck_Parts_Accessories&hash=item258b273622 http://www.ebay.com/itm/Ford-Exhaus...r_Truck_Parts_Accessories&hash=item2c80514e6b
The reason I suggested looking at the dimensions of the Torino headers is that, first, they are available, and second, having had both a Torino-Bird and a 78 Grand Marquis with the 400 at the same time, the layout ahead of the firewall is virtually identical. The biggest difference between the two platforms is the length of the wheelbase. Both have the steering box behind the crossmember, and that is what causes the bulk of the clearance issues.