Did you try to loosen the two end bolts? They may be over tightened. I don't have the torque specs handy. On the 250 and 292 the two outside bolts are 20# and the center bolts are 30#
Tube header flanges are simply too weak to be clamped without replacing the guide pin with a stud. It's real easy to get leaks from over tightening tube headers. At least it is when you replace all the bolts and pins with studs/nuts(w/fine treads).
