Tim gave you a great link! 9.595 was the stock length of all non-adjustable ball and ball push rods for 390 engines. You could get them .030 shorter and .030 and .060 longer back in the day. Not sure if you still can. Some motors from the factory actually had different lengths in them. A non-adjustable is just that, non-adjustable. The amount of “load” you put on the hydraulic lifter depresses the cup down. That is why you go to zero lash and then turn anywhere from ¼ to ¾ turn down after zero lash. The lifter must be depressed to work properly.
It is critical to have a good engine builder that understands this in a non-adjustable motor. If the heads have been decked then the whole geometry must be shorter. Hence the shorter push rods. But valve stem length and even the valve seats in the head play an important part.
I have run rebuilt and run many 390 and 428 engines over the years. It does cost a little bit, but a great upgrade is to the adjustable system that came on the 427. It is a direct bolt on, under the valve cover, and no one knows. Uses a ball and cup push rod. I have seen many of these for sale on flea-bay. Lots of FE owners are stepping up to a roller tip adjustable rocker arm set up. Dove and Erson sell these set-ups. Be prepared to spend about $600 or $700 dollars.
A 427 adjustable system can be very affordable and then you know you have proper adjustment rather than just putting in a longer push rod to cure a lifter rattle. And no one can see it. And they are easy to install with just removing the valve cover. Be careful as you pull up on each push rod. Sometimes the lifter will want to come up to. And then it falls out in the valley under the intake. I have been to this rodeo before and it can be fun to get it back in its hole!!
Rob