I had one 340 motor years ago that would eat the #5 exhaust lifter after 20,000
It did that twice before we figured out that the block lifter bore was NOT machined correctly That customer and friend ended selling the car after the second failure with out fixing the block
My message is to always break the cam in the block your going to use it in and be very careful about reusing a cam and lifters in another block
I don't have a accurate number on the total motors I've rebuilt and blue printed but well over 200,I have had three or four of those blocks machine for and fitted with lifter bushings