I used the one from 440 Source
then you need a 10" dial Caliper that you can use to measure the length. I ordered that off amazon. Worked out OK for me, took all my measurements, then called trend and they had my pushrods made and delivered within the week.
https://store.440source.com/Adjustable-Checking-Pushrod-RB/productinfo/200-1098/


1963 Belvedere (mine)
1969 Roadrunner (wife's)