Spacehog - I'm doing a similar build to yours, only for the Saiga 308. BTW - your/this thread has been very helpful.
Could/would you be able/willing to take some more photos to post?
Specifically, I'd like to see how you modified/cut the Saiga sporting stock and then attached it to the rifle tangs. And possibly how you had to modified the Kushnapup stock to fit. I, like you, do not want to saw off the tangs and prefer to leave them in place.