I believe your engine has split manifolds on each side with two down pipes - effectively making them "tri-wye" headers, and I doubt if an aftermarket header, especially a four-into-one will result in any improvement, and they are likely to be a lot less reliable - longevity, bolt retention, etc.
I know the earlier 4.5 V-8s had this exhaust arrangement. Not sure about the 5.6, but even if it doesn't, headers are not likely to be of much benefit. To be effective headers need more valve overlap than most OE cams have, and to be effective in the rev range of you engine, the primary pipes on a four-into-one need to be at least 36" long. Most aftermarket headers have primaries that are too short for the production cams and rev range.