Are we talking intake or exhaust manifolds ? The lower intake is exactly the same for all years, even the 95 cars have an 89 casting # piece. Exhaust manifolds will bolt up the same, but the 94 - 95 R/S manifold has a different outlet location, and the EGR tube hookup is changed as well. The heads have different valve cover mounting rails, so I think that you need to use the 89 - 93 covers and gaskets with the earlier heads. I've heard that the coolant passages may have been revised slightly on the later heads as well, not sure on that though.
There are many people running early heads on late cars, and vice-versa.
cheers
Ed N.