Hey H-T, I was just wondering if a 99 civic cat will bolt on perfectly onto a 96 integra GSR? Searched.

 

Doubt it.

GSR cat on a 96 is roughtly 17 3/8"

Civic cat is around 16 1/2"

Most notably that GSR cat has both O2 sensors built into the converter unlike all other models of integras, with the secondary on the cat and the primary on the downpipe.