I think Schumacher is the only one that makes them, they appear to be sold out
'73+ A,B,C,F,M,J-Body 273/318 or 340/360 Replacement Kit
The 318 mounts work, you just need to add a spacer between the ear on the block and the mount because the mount is wider on the 318. I suppose you could also split the 318 mount, removed the 1/2" or so of extra width and weld it back together if you were so inclined.