It's interesting that the '60-'64 pushrods are the same length as the '54-'55 y-block pushrods per JMs chart.
I've got both lengths of the earlier style solid steel 'knitting needles', Ray, but none of the later tubular style to offer you.
I don't know about the appropriateness of using the longer length pushrod you've got as a substitute. As you noted, there is the adjusting screw on the rockerarm to provide leeway for assembly variations, BUT. The whole issue of rockerarm (valvetrain) geometry is what we're dancing with here. I've always used 'what-you're-suppose-to-use' for the application and never gotten into the nitty-gritty of it as you do if you're building something that's going to be a serious performer and operate at raggedy-edge rpms.
Ted Eaton has an interesting article on this stuff on his web site. See
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York