Like he said, ported exhaust manifold, ported Y and if you don't need it, get rid of the CAT. It is likely clogged.
You may need a muffler, but you may not.
The real issue after that is done, is the intake and flow.
I built a 2.8 to 132 hp at the wheels. It took porting the heads and all of the above. I also carb'd it to get more fuel mixture into it.
In short, though, the ported exhaust is the quickest and most efficient improvement.