Interesting. ATK is also doing business as the Powertrain Company and, by coincidence, is located in Oklahoma City. I called them. Here is the deal.
If you want a 351C, then it comes in two flavors: $2875 or 2120. Other than price Bob the salesman did not what the specific differences are between them.
If you want to buy one, then they don't have them in stock. They then check with their vendor, who is located in DFW ( Grand Prairie--I think). They then have to locate a core and proceed to build the engine. If they can't find the core, then the date of delivery is off in the very indefinite future. Bob did agree that the engine would have to be over-bored to some degree, but that would depend on the core condition.
So, it is not a true option for someone wanting a stock 351C and such a person would be better off pursuing an engine build from a reputable engine builder local to them.
Warmest regards, Chuck Engles