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Published weights were approximates with many members reporting their boats were more.

For only 10-15 lbs. many will say you are good. Seal ALL holes, cracks and exposed fiberglass.

A 6" setback bracket is the reason your stern heavy. Moving a 40 hp motor that far behind the transom is shifting the static trim way way back. I would think twice on stressing a transom that's 45 years old like that.

I would remove the bracket, remount the engine and splash and observe hull's static trim.