YES 5.9 Magnum manifolds work in early A bodies. Here is exactly how!
I measured all of them and test fitted many of them on our Mopar engines. Some of the large hole truck/van manifolds will work on the passenger's side but exit very close to the firewall and will introduce "heat" into the firewall. I found that the JGC manifold (96/97/98) fits perfect, even better than an original 318 manifold. Just remember that just because a manifold came off a magnum engine it doesn't mean that it has the 2 1/8" exit hole.
Remember that the magnum engine (as it's known) was made for many years AND all or most of the manifolds on these magnum engines were referred to as "magnum" manifolds. BUT.......... as it turns out, most of these "magnum" manifolds were NOT the large hole (2 1/8") manifolds. There were some large hole manifolds made in 92/93 (Dakota manifolds I think) and some made that came on trucks and vans in different years. I researched these manifolds for many years. I purchased many, many so called "magnum" manifolds and conducted many experiments. At one time I had as many as 30 sets of manifolds.
I measured all of them and test fitted many of them on our Mopar engines. Some of the large hole truck/van manifolds will work on the passenger's side but exit very close to the firewall and will introduce "heat" into the firewall. I found that the JGC manifold (96/97/98) fits perfect, even better than an original 318 manifold. Just remember that just because a manifold came off a magnum engine it doesn't mean that it has the 2 1/8" exit hole.