1994 Chevrolet Truck wont pass smog

I have been trying to get my truck to pass smog. It wont. The guy at the station says its running to rich. Another guy said, it was a vacuum leak, another guy said, I might have a cracked exhaust manifold.
A few years ago while I was having a cat installed I recall the muffler shop guy saying that the exhaust manifold might be cracked. I do get a check engine light, often. It seems like it happens more when I down shift to slow down. I purchased my truck new, it has well over 350,000 miles. Odometer stopped at 280,000. That happened like 10 years ago.