I have a 87 notch 2.3l we got it working fine.A few weeks later i heard a loud popping noise coming from the exhaust.Like loud backfire.I have a 94 gt rear end and front 5 lug swap.There is black smoke coming from the exhaust.I hear some noise coming from the bottom where the cats are at.Some knocking as well coming under the hood.Any ideas what all it could be??

 

Sounds like you have a cat that is coming apart. I had the same problem on a old pinto I had. There are two options get a new one which is pricey or take that one off and use a broom handle and bang it till the cat is hollowed out. I don't know how strict your inspections are where you live but here if a mechanic see the shell of a cat he is good with that. Good luck