Is the locating pin out of the block? Edelbrock makes a set of intake gaskets for the Magnum, it doesn't include the plenum gasket and is a lot less expensive, Summit doesn't carry it. I got mine at Shuck's, had to order it.

I would dry fit the intake on the engine with the head/manifold gaskets on and see what type of gap you have on the ends. Off hand I don't know what that gap should be, but someone could tell you if it is excessive. The manifold bolts only take
144 inches of torque.