Thanks guys, the correct screw is the one 1955-cj has the picture of, the length cannot be more then 3/8", 1/4 would be the safe choice.
I re-measured mine (I mean Bubba's) last night and it was 7/16". That was a $455.22 mistake (had to have it recored) that I will not soon forget.
Macrisel you may be safe by 1/16 of an inch, or not....
One more bit of education on radiator installation, tighten the hose clamps lightly and re-tighten them if you have a leak. The top and bottom ports on these old radiators crush or warp very easy.