This isn't my forte, but I'm going to guess that part of the problem might be that the RB Models parts are for a DShK model 1946. The Tamiya JS-3 should still be sporting a DShK model 1938. Obviously the clamps holding the bolt carrier assembly to the barrel should line-up. I don't know what's going on with the receiver, but the photo you posted doesn't look right.
That's all I've got...