YES 5.9 Magnum manifolds work in early A bodies. Here is exactly how!

Crossing my fingers the manifolds I pulled off a ram van with 2-1/8" openings fit. Also got the pass side off of a JGC, also 2-1/8.
The passenger's side manifold off of a van will usually fit but the design of the manifold will cause it to exit near the fire wall introducing a little heat into the floorboard. But they will work. I never tried the driver's side manifold off of a van because vans have tons of room on the driver's side compared to an A body so they weren't worried about cramped spaces.