The first link tells you the batting average (AVG) and isolated power (ISO) each year in the AL for both close and late hitting vs. non-close and late hitting since 1950. All data from Retrosheet.
AL Close and late hitting vs. non-close and late hitting
Now the same thing for the NL
NL Close and late hitting vs. non-close and late hitting
This next link simply shows the differences in each stat between the situations in both leagues. Numbers in red are positive or zero. Those pretty much stopped in the 1980s. That is, since 1990, AVG and ISO have pretty much been lower in the close and late situations than otherwise.
Yearly differences of each league
The graph below shows the annual difference in AVG in the AL.
Now for the annual differences in ISO in the AL
The next two graphs do the same thing for the NL.