Your mechanic is an idiot. Or he just doesn't want to do the work because they are a pain in the *** to install, require dropping the K. Also, for a good set that fits right you are looking at $1k and that is just for the headers themselves and the midpipe. The cheap ones you see like the Pypes/Pacesetters are junk, the BBK ones have a big kink in one of the primaries to clear the steering shaft so that leaves the SLP as your best "good" one and the one I would recommend.
Also, for a NPI 98GT you are not going to see much at all from a longtube install and as was mentioned above, they might not be legal depending on where you are located. First thing I would do for a NPI car is look into a PI swap if you don't have that already. Then worry about bolt ons/suspension and look into longtubes as one of the last things you do.
In all honesty, the factory manifolds on a 4.6L are not restrictive, at least not enough for them to be worth swapping out IMO. It isn't like the old pushrods where the factory headers were just garbage and required a R2 as soon as possible.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.