Do the batsmen of 70's and 80's deserve some grace marks when we compare them with modern day batsmen ?

Good question, great stats! But the point is yr view of competitiveness, in my opinion, is not fully justified. In those days Indian team used to be treated just as Bangladesh is being treated now. The competitiveness was restricted to the ashes series between England & Australia.Yes with regard to protective gears, yes the batsmen of 60's, 70's n 80's do deserve some grace marks. But in the case of Gavaskar, the helmet he used to wear had an element of protectiveness and so no special marks for his 13 centuries against what u call the most deadly bowling of his time.