There is a big "X" cast into the block on the front of the #2 cylinder. There is also an "X" on the back of the block but I don't remeber where it is at the moment and my only "X" block is at the machine shop.
It also MUST say 318 in the casting number (damn near got into a fist fight with a liar at a swap meet trying to sell a 360 block as an X block by saying he ground and polished the X off...the dead give away he was a liar was the fact it had a 360 casting number and 360 drivers side motor mount) and have the 318 drivers side motor mount.
I would never take a 340 past 4.100 unless it was sonic checked...and then I would use some block filler. The Chryslers have RELATIVELY long cylinders and even though there may be enough wall thickness (as per chevy rules of thumb) I would want more support as the piston goes around BDC. A stroker crank would magnify the issue.