I was told to measure from the ball end to the bottom of the cup. So I had to measure the depth of the cup then subtract that from the overall length to get to the length needed to order a set from Home - OE Push Rods - Manufacturer & Distributor.
If you read a lot of threads you will find that few if any have had luck using off the shelf pushrods with 273 rockers.
I made a measuring tool by cutting piece out of a wrong-sized original pushrod and tapping the two remaining pieces to join them with a piece of 1/4"all-thread with a locknut.