Based on what I have learned about exhaust gas and velocities, Ford went to a "larger than necessary exhaust manifold on the 4.0L to beat emisions on the cheap. The 94TM head casting exhaust ports are the exact same size and location as it's predicesor the 2.9L; while the 4.0L manifold was way oversized and offset but matched up.well to the 95 head casting.
Good header brands are JBA and Borla, Pacesetter also makes headers but you need to get the coated version, if you want them to last more than 5 years.
I have 2 sets of JBA triple nickle played headers, going on 22+ years of use, just like new, they were even exhaust wrapped, which voids the warranties.
If you have a 94TM or 98TM head casting, 2.9L headers are a SOLID option but their "Y" pipes are built for dual exhaust at the "Y" pipe to converter flange.