Harmonic Balancer and Timing cover

trying to finish my rebuild of 340 and running into a complication. During rebuild i found that the water ports on the timing cover corroded 2 holes which caused the head gasket type symptom of fluid switching. I purchased from summit a Dorman replacement 635-400. When trying to reinstall my old harmonic balancer it makes contact with the cast timing marks. Has anyone ran into this problem? it would look as though i would need a less than 7 inch dampner