I have been told that the factory headers are pinch to fit thus restricting the flow.Is this true?
Dunno. But before you start playing with headers, look hard at the driver's side cat.
From reading others, it seems like that's not the main limiting factor on most 5.0Ls anyway (general maintenance, gearing, torque converter, and heads - in that order!- seem to yield much more improvement.)
I do know that taking the 5.0L in Sweetie's Cougar from the poor state of maintenance it was in when I bought it to now has improved its performance - and its gas mileage! - quite considerably (from about 12 MPG to 23.2 MPG at 70MPH cruising ... )
And that's with stock 3.08 TL, stock torque converter, stock tranny, and stock tune on the PCM.
The headers help some (the factory ones, anyway.)