Welcome to MustangWorks!!
Yellow 98 GT's are awesome. Wanna really feel a part of something big, check out
www.yellowmustangregistry.com. It is a whole club dedicated to yellow Mustangs, and they put on cruises and events all over the country.
Your antifreeze problem is common, the 4.6 has a plastic intake manifold. This is all well and good except for the front which passes hot coolant from one head to the other, and they get cracked, either at the thermostat housing or across the runner behind the alternator. I have had to replace the manifold on my 97 GT and on my 98 GT. My sister-in-law had it happen on her 97 T-Bird too. The good news is Ford has figured this out. The new replacements (my 97 got another all plastic intake) have a metal runner on the front, aluminum I think, so when it is fixed, it stays fixed.
Ford had a recall on this manifold, chances are you can get it fixed under the recall. Check with your local Ford dealer. If not, the new manifold is about $250 and you can replace it yourself. Be careful to take all the parts off the old manifold one at a time and set on the new one, so you know where they go. Then when the old one is off, transfer them from the new one to the old one, then start replacing the manifold. Its a big job, but it's not really hard.
Be sure and post some pics soon!