Noob needs help!

[grayman]

If you get a 3 inch v8 cat back (LT1 - good choice by the way) you will need a new cat that is 3in/3out. your stock v6 cat is 3in/2.25 out.
The LT1 catbacks also come with a flange on the s pipe. cut that flange off and the new 3in cat will slip right over it...suggest using stainless steel tbolt band clamps rather that cheap u-bolts.
The v8 LT1 catback will also not come with an O2 sensor bung. you will have to have one welded in the the S pipe or get a cat that has a single rear O2 bung. visualy check your current CAT to see if it has an AIR TUBE or not before you order cat.
Yes, single cat system is what you want.
I am not positive...but your '98 may not pass emissions if your state/car uses OBD2 testing. You will know because when you run car with new system you will set of codes. at that time you will need sims.
To tone down exhaust further if it is too loud for you a dynomax bullit muffler can be added to the mid pipe. 12" or 18"...18" is pretty much the entire mid pipe section with just the catbacks mid pipe ENDs clamed/welded onto the muffler. The dynomax is a straight through perf/fiberglass filled muffler that will not reduce performance much if at all but it will quite down the overall system as well as inhibit rasp and give lower overall tone.

moving up to a full 3" system IS very do-able...but is much more involved than just getting a dedicated (built for your engine/car year) v6 catback. So the cost ov V8 sytem will be more and more hassle as well. You could still add a 12" dynomax bullit to after market v6 catback to further tone down the system if your cops don't like you any more because ANY catback system you go with WILL be considerably louder than stock whenever your foot start pushing down on the throttle pedal.
I learned about the LT1 v8 sytem here (thanks ELLICK) and have one sitting in a big box in my garage RIGHT NOW!!!!! waiting for me to install it this weekend. I will be making a full webpage for it very soon.

good luck with your project.

[grayman]

Update regarding 3inch system:
something had been nagging at me so I just went and checked it out. If your 98 has welded y pipe to cat from factory rather than older models that have flanged cat on both ends....and you choose to go with a v8 catback from 93-97 then...my 3/3 magnaflow CAT has end collars that are sized to slip OVER 3" exhast pipes. once the flange is cut off of the LT1 s pipe the 3" cat will slip over it...then use a clamp or weld after system is all fitted. For the y pipe...the factory cat is some kind of custom...non aftermarket type sizing...the stock cat's collar slips IN the y pipe pipe and has a smooth, clean, very tight factory machine weld. the outside diameter of the y pipe near the stock cat is about 9.5 inches outer circumference. the outer circumference of the aftermarket magnaflow 3/3 CAT is about 10 inchers. SO, I will be cutting the Y pipe just past the CAT-Y pipe weld and then slipping the magnaflow collar OVER the y pipe and using a T Bolt band clamp.
In other words the only modifications to an LT1 3 inch full sytem is to cut the flange on the new S pipe, and the tail pipe hangers are different...havent got there yet...but easy to fab or order LT1 hanger from parts store.
You will still need to locate an O2 sensor bung either in the S pipe or get a cat that has rear O2 boss already in it.

The cross member that goes under the S pipe may have to be shimmed with washers or something in order to add clearace for the fatter S pipe...easy cheesy.
the welded support mount for cat will have to be reworked or replaced. I plan on using a cheapy u bolt and some metal bracketing to bridge that mount.