How about a flat spacer between the biscuit and the k frame. 1/4” aluminum plate with a hole or slot in it. Put one on both sides. I’ve done this on small blocks for header clearance.
This is what I was first thinking but it would need to be quite a bit thicker. Probably 3/8" to 1/2" to account for the movement of the rubber mount. I going to end up trimming the crossmember like they recommend.
