I'm in the process of installing a newer CD head unit (from a 2001 F-150) into my '97 F-150. Today I removed the steel bracket (without having to cut anything) and test fit the unit into the opening. It fits perfectly. In order to remove the steel bracket without cutting, you will need to remove about ten screws from the surrounding dash panels and eventually you will remove the three screws that hold the steel bracket in place. Two of these screws are threaded from front to rear (that is, you have to reach into the cavity that the HU comes out of and feel them with your fingers. You can easily get a wrench on them.
I've spent some time looking around for wiring for the HU as well as the 6CD changer that is also from a 2001 F150. I think that this site
www.radiosandmore.com has everything you will need. I'm not certain if your '98 HU has the older style plugs or the newer "world plug" style but it will be obvious once you look at them and the information on
www.installdr.com ( or just google the part number from your HU), in any case radiosandmore sells a simple adaptor (part number ADP1) for $25
http://www.radiosandmore.com/prod_detail.asp?pn=ADP1 that will keep you from having to hack up the vehicle wiring.
good luck, hope this helps.