351W flywheel+head question
Early SBF, including the 302's, up until 1980 or so and all 351 share the same factory imbalance, 28 oz. If there aen any custom changes, obviously that brings up a whole separate set of issues.
After that, 302's (5.0 liter) went to the 50 oz balance. Your flywheel/flexplate weight offset is not a problem.