Don't look too hard between the valves on the end cylinders, if you look hard you will find cracks there and not run the heads. Literally 9 out of 10 mag heads I have looked at have cracks there if you look hard enough but I have never seen one cause any problems.

I can, or any competant local machine shop should be able to machine the proper bolt holes in your new heads for a lot less than the price of a new manifold.