Super six

Yes, that crack is a major problem. It will let exhaust into the intake tract and the engine will run very poorly. Guessing you've got one of the super-lightweight welded intakes (weld bead clearly visible around the upper perimeter of the manifold, and № 4 runner slightly curved rather than straight parallel to № 3). This what you see is one of the downsides of those intakes; the hotbox area gets the most severe thermal cycling and eventually fatigues/cracks. You could try and weld it up, but you're dealing with weakened metal in that area and the odds are it'll crack right next to the weld or elsewhere in that hotbox area. Probably best off finding a different intake (or making one; see the parallel 2bbl setup article. Photo documentation of a very well done such conversion is here.)