The best way would be to replace the whole system to 2.25"

But using stock, you'll have to adapt from stock size (1 7/8"?) to 2.25", either with welded adapters, or a couple clamps. I wouldn't call it chinsy, but I wouldn't call it perfect either.

 

sorry to thread jack, but without the cat will it affect the sensors? i have an 89 now, but in my 86 didnt have a sensor after the cat. I just took off the cat and got a glasspack and ran pipe out the back and it sounded good. But it didnt sound throaty until i took out the cat so thats why i was wondering about the sensors

 

I'm working on this kind of setup for my rig. Cat-back Flowmaster 40 2.25", and I'm keeping the stock cat for now. I'm just gonna 90-degree-bend it out in front of the rear wheel. Hopefully that'll be enough to avoid the fart sound! In time, I'll do the rest of the exhaust in 2.25".
This is what I did to connect the 2.25" Flowmaster to the 1.75"-ish pipe: 1.75" ID to 2.25"OD adapter welded to a 2.25"OD to 2.25"ID adapter welded to the muffler itself.

If that was confusing, just ignore this post and listen 4bangin. He knows what he's doing!