connecting rod and harmonic balancer question

The 2899496 rod is a floating pin type,and only came in a '68-'73 340's..the 3418645 is a pressed pin type found in 318/360's...

Not necessarily true. I've seen both forging numbers in the floating configuration. I've had multiple sets of each. Could it be that someone converted the 3418645's into floating pin? Yes, but I've seen so many that were floating, I doubt that every single set was converted over. Mopar is famous for using whatever they had laying around.