The stock 1963 to 1965 BB stock rubber mounts will have two studs(3/8x24 thread I think) that go through the steel K member mounts with one larger stud that fits into the steel block bracket
All the BB mounts, steel for the block and the rubber mount that bolts onto it, for 1963 to 1965 are the same for 361,383, 413 and 426 W which is the same as the 440 block mounts on the blocks
There is a right and left side steel block mount also, if I'm remembering correctly
They don't inter change side to side, they fit one side only