Big block A body spool mount K member?
Either that's the old DC K member or one someone made from a small block spool mount k frame. I've made one myself. Essentially they cut the small block mounts off and moved them to allow for positioning of the big block. You can't just plop a big block on an unmodified small block spool mount k frame without cutting and moving the mounts, OR you use Schumacher mounts, which, instead of cutting the K frame, they 'cut and move the motor mounts' themselves instead of cutting the k frame. Either way works. Here's a pic of the one I made.