How hard is it to find CAT 627E Scrapers with a 3406 for the rear engine?

I was told the 627E is a much more productive machine with the 3406 rather than a 3306 for the rear engine, so the hunt is on! Trying to put together a good fleet of push pulls without breaking the bank. Can anyone add some information about them? What Series would use them?

Thank you,

Iron Man

How hard is it to find CAT 627E Scrapers with a 3406 for the rear engine?