Depends on where the leak is. Is it the manifold to head area? Is is a broken manifold bolt? Is it the manifold to exhaust pipe? I have read some of the stock manifolds develop cracks.
A leak could be several areas with very different fixes. Also the 460 manifolds I think used to have usual manifold to exhaust flanges with a gasket. Later models have a cone and cup design which in my opinion is prone to leaking due to heat(like towing) softening the pipe and allowing the cone to shift.
If it is a manifold to head leak, then removing the manifold and cleaning the area and using a new gasket is a good idea. Be SURE to use some sort of penetrant on the bolts. If the manifold has been in place for awhile the bolts may be rusted all to heck and you have a good chance of breaking some, and then you have a real PITA.
I would bet someone makes headers for just your swap. If it was me, I would probably go with a good set of headers. But then you are probably not suffering from any lack of power as it is so if your stock manifold is in good condition, it may be just easier to keep it.