Common to have shock tower cracks created by stess on V8 cars, the extra (not that much extra) weight of the 351 windsor, would not make a huge difference.
They only weigh about an extra 25 or so pounds i think.
Have the crack proffesionally repaired by MIG welding and install an export brace if you don't already have one.
Also check upper spring insulators and shock absorber mounts and upper control arm shaft bushes. Check the bolts that go through the upper shaft, through your shok tower also.;}
