Which motor mounts do I need?

We are getting closer to putting the 408 in the Duster. My cousin mentioned that the 318 motor mounts are different then the 340 and 360. We looked at the two blocks and there is definitely a difference in the width on mounting to the block. I assumed I needed to buy the /6 conversion mount since this car was originally a /6 but measured the perch distance and it came to 20" so I believe that means it's a V8 K member. The guy that had the car before me had a 383 in it.

I figured I could just go on ebay and buy motor mounts but what I finding say fit 318/340 / 360 and I know the mounts on my 318 wont fit my 360. I am not sure what do order and I don't want to get the wrong mounts.

If any one is able to point me in the right direction that would be great.