Differences in Exhuast Sizes Through The Years

Well i hacked my exhaust off long ago and put a turn down on after the muffler and i kept what was left for a rainy day. Well now my GF wants to redo her exhaust and told her that i had the back half of my exhaust with a chrome tip welded on the end for her to have. So she bought a thrush glasspack as a cheap replacement and im wanting to install it for her by just clamping it on there like i did mine. Now here is the issue.... All my original exhaust piping is 2.25" and i went to measures her piping today and it's like 2" in some places and then smaller than that as you continue to the tip, so the muffler we already purched has a 2.25" inlet and a 2.25" outlet and is 25" long, so i was wondering did they use different sized exhaust piping throughout the years, because now im going to have to figure out what to do with it. Because when we got her Cat replaced, the person who did it welded a new peice from the cat to the muffler, that is about 2.5" so now i've got all three of those different measurements to deal with.
Just need some help on this.

Welding is what i want to eliminate. Don't have the money right now to get it done at an exhaust shop, so i wanted to just clamp everything on like i did mine. But im not so sure its going to be that simple with hers...

Clamps will work ok, the're just not as good as being welded. and you'll have four clamps instead of two.