I know there isnt much difference. I may be picking up an 89 L98 car in the next week or so. I was thinking of putting an SLP Loudmouth under it. Would it bolt up just like on a 4th gen, or would there be fabrication involved in trying to put it under an IROC

 

Where they connect behind the cat is different, easy solution is to make an adapter piece.

Also I think the rear hangers changed, easy soulution get some 4th gen hangers.

 

It will take some miner mods but it will work.