As I understand it the cracking in the usual places around taillights, A pillar at the lower windshield and rear decklid at the curvature near the gills is predominantly due to work hardening of the lead filler over the years. It would be prudent to remove the lead and refinish these areas.
If your vision of originality is to maintain the original (non adjustable) chassis cross brace then some welded in reinforcement would likely help prevent future chassis flex and cracking.
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