It can vary.... I just went through all of this a few months ago in ongoing research of this topic. I don't have my numbers handy.. but.. the problem is there is NOT JUST ONE 4 ROW 66/67 Rad. My 66 SS Convert. Automatic., that I just sold.. had an Original 4 core HD rad in it, never out of car.. the top tank was no diferrent in Size/Width from the Standard Radiator in my other 66's., but it was 4 row. I have now seen and sold 4 different 4 row Harrison 66/67 SS Rads. and it varies.. ie.. HD because of L-78 vs. HD because of Factory Air vs. HD because of ......
Then I sold my other HD 4 Row Automatic Harrison Rad. and the Top Tank for that one was CLEARLY wider.. about 3/12" wide. I have the measures at home.
Another Member and I have been working on this research for a bit...
That all said.. I am going to assume you are verifying the width of the wider version 4 row HD tank and i think it is 3 1/2"