Do you have to swap K members for /6 to SB?

Just thinking to myself. If this /6 ever dies (which it probably won't any time soon) do I HAVE to change K members to a small block style or can you drop a small block in and fab/weld motor mounts to the /6 K member?. What are the problems you run in to? I have some nephews that are Mopar fanatics and have access to 318's and 340's that can be had for a reasonable price.