Probably. Ford's 352 is just a smaller bore 390. There are two possible issues though. One is that even if they are in the same engine family there can still be differences in shape and configuration of the manifold. The other is that somewhere in the distant past (but I think before 62) Mercury used a different engine design. It was more similar to a Lincoln 430. If your Mercury has that engine design the 390 exhaust manifolds will not fit.
I think I would take a chance.
