IN your subject, you may want to refer to exhaust manifolds...some people may immediately think intake manifolds. As far as I know, they SHOULD fit....but I dont think they would offer any benefit compared to a set of 289 manifolds or some shorty headers.
The design on 351 manifolds is about same as in 289 Hi-Po, so they are better than std exhaust manifolds of 289. If you like your engine quiet they are good for you, but if a little more noise isn´t a problem go with headers (like hedman, don+t cost much but do the job ok)
The guy at the parts store might have been thinking of the Hi-Po manifolds. But considering the cost and that you would have to modify the pipes, I would go with headers instead.
Years ago, Hot Rod magazine ran a couple articles by Ak Miller. He built and dynoed a 289,(or 302, I can't remember) with a mild cam, intake etc. He removed the stock exhaust manifolds and replaced them with 351W manifolds from a 69 Mustang. The result was an honest 5 horsepower gain, from just above idle to 5500 rpm. But I think at least one of the manifolds has an odd bend at the end, where the H pipe bolts, so the 289 HiPo manifolds would be probably be the way to go, and since they look almost the same as the 351W manifolds, power increase would be similar.
The design on 351 manifolds is about same as in 289 Hi-Po, so they are better than std exhaust manifolds of 289. If you like your engine quiet they are good for you, but if a little more noise isn´t a problem go with headers (like hedman, don+t cost much but do the job ok)