The pushrod diameter is the same. The end of the pushrod that contacts with the rocker is the same.
It's the diameter of the cup in the lifter, plus the lifter end of the pushrod, that are different. The earlier pushrod has a clearly visible necked down portion that fits into the smaller cup on top of the lifter.
They ARE different lengths. Overall length of the early one is 8.645", while the later model pushrod is 8.575"
Pictures is late pushrod. Find a picture of the earlier pushrod end here:
http://www.rockauto.com/en/moreinfo.php?pk=216005&cc=1435010&jsn=493R.
All the early, pre 1968 or 1967, had the 1/4 inch ball pushrod ends for those lifters that where different lengths.
I wouldn't bet money that Mopar went to the later lifter cup and same size pushrods on both ends so some of the dealer wrenches wouldn't install them upside down and pierce the rocker arms under warrenty
I did that on a 1965 413 I put together for my wife 1965 Dodge Coronet 500, install one side correctly and did the opposite on the other side, I ended up punching through three rockers arms going up the old highway from Mesquite AZ to St George, UT back in the early 1970s
Luckily the local dodge dealer had them in stock and some new lifters for that year also
The retaining clips in the lifters had got knock out of two of them and ended up washing down into the oil pan, a good stick magnet got those two lifters out and really help installing the new ones