I just replaced the cam in a " rebuilt" 391 FT, and it had 3 different length pushrods. 8 at one length, 4 and 4 of other, different lengths! They varied by an 1/8", and two of them bottomed out in the lifters and wiped the cam. It is surprising, but the power stroke event has a way of slamming a valve shut even if it is held slightly open! Pushrods can buckle a little.
It is good to check these things if you were not the one that assembled it last time!
(It did not need the odd lengths this time because the heads were properly done, but sometime in the past it must have had a goofy valve job. It was all just wrong)
As mentioned, also check the oil holes in the shafts.