You can still get pushrods. Truth be told, the aftermarket replacements are stronger tube construction as opposed to the solid rod construction of the originals. The length of the pushrod depends on the style of lifter. Early to middle 55 and back solid lifters motors used a taller lifter, looks like a milk bottle. And in fact many people refer to the those style lifters as milk bottle lifters. Those motors use the shorter push rod. If a 56 and up truck motor used a milk bottle lifter from the factory, I have never seen one. Later 55, and all 56 and up truck motors used a shorter lifter, resembling the hydraulic lifter used in the Power glide car motors up to 55, and 56-62. Those motors used a longer push rod.
Here is the catch. Sometimes there is a mismatch between the ball on the rocker adjuster and the cup on the pushrod. The ball being too big for the cup. What happens is that the ball is contacting the just the rim of the cup and pounds on the cup until it splits.
If you are replacing pushrods, inspect the condition of the ball adjusters, they should be relatively round. use a little red or blue dykem on the ball adjuster and the cup of the pushrod, and run the ball adjuster in on the pushrod by hand. Take a look at the register marks where the dykem wore off. You want the ball in the cup, not just on the rim.
