What type of build up is it going to be? Budget or...? The factory stuff is pretty stout so if its just for street you can probably re-use whatever is viable beyond the rotating assembly.

There are inexpensive stroker kits available from anywhere though - Hughes, Mancini, Marsh, Indy, Summit, etc.

If you go with new iron heads the EQ are hard to beat but not sure they are available anymore? Edelbrock makes Magnum-specific heads, or at least they used to.