That's why I said likely. I'd bet case it's out of a duster...
Given your reasoning then he could have gotten good coin for a 340 and bought four 351s or 302s ... many will scoff at it now
My buddies kid found about a dozen mopar V8s and trannies on a piece of property out in the boonies in a dirt floor shed. Along with a mid 70s newrport wagon, and a pretty solid rust free 70 318 duster, with a Go Wing sitting on a shelf in the shed.
They payed him to clean the property up. In this day and age google makes everybody an expert. All automatic trannys, mostly 318s, a couple big blocks, and one 71 340. He wasnt interested in selling individual engines and trannys wanted it gone as one lot, found a guy in montana who bought the car and all the engines and transmissions for $3500
C body was filled up with scrap steel on the property and hauled to the scrap yard. I told him i coulda used the front discs off it for a second A body project.
Heck aa far as 340s go, i found one at my local boneyard inbetween the framerails of an early 80s dodge pickup. Intake and heads were gone. Cylinders filled with rust. A shame i didnt see it sooner or i woulda nabbed it.
My long winded point is that they are out there, if you really want one. I have seen guys LS motor mopars too. A simple K frame swap, and put a mopar engine back in. No harm done.