Header, and yes the early 90's (atleast, im not sure how the later model ones work) tubular header will bolt on.
But you will need to modify the exhuast.
The only things I needed to modify when I did my swap was the EGR tube and the O2 sensor bung. The header I got came from a '97 Ranger and on those, the O2 sensor is located in the exhaust pipe not in the collector so I had to drill a hole and weld a bung in for my O2 sensor. I snagged the EGR tube from the Ranger when I pulled the header. I used the Ranger end at the header and welded it to the Mustang tube. It took a little trial and error fitting but it worked out great. The Ranger had a smaller diameter tube which slid right into the Mustang tube which made welding them together a breeze. Other than that, it bolted right up. I paid 40 bucks for the Ranger header and tube at a local yard, a couple of bucks for a new manifold gasket, and that's all I have in cash in the project. The rest was my time and labor.
I think earlier Rangers (93 and 94 I know for a fact are this way) have the O2 sensor bung at the collector so the only modification would be to the EGR plumbing.