you can replace the up tubes and the donut gaskets and be fine. I would DEFENITELY have some Lock Lube handy to get the bolts loose... The difference in the flanges are for the 99 year was split. The early 99 usually take the smaller bolt diameter split, if you own a 99.5 and newer, get the bigger one.
Symptoms- Lack of power, exhaust odor, exhaust noise from rear of engine at idle(bad leak) or on accell (normal leak), excessive black smoke...varies.