Yes, big block Mopars are Y-blocks. Doesn't have anything to do with the spread of the V (which is 90 degrees for any even-fire V-8). Has to do with the block extending below the centerline of the crank (i.e., the lower leg of the "Y"). A poly 318 block ends at the crank centerline, just like a wedge 318.
As for swapping LA heads onto an A engine, I've never done it myself, but apparently some have:
https://board.moparts.org/ubbthreads/show...p;vc=1&nt=2Also, poly 318 heads flow better than 318 wedge heads:
https://board.moparts.org/ubbthreads/show...rue#Post4565497(Be sure to read all the posts in the links above.)
Here's a pic of a T-bucket with a poly 318 I pulled off the internet a while back:
