Nope... I remember seeing those kits and they did actually specify mismatched trunions..
Whether or not you actually got one before the ads were corrected, it was a a big issue and a lot of folks were upset....
The fix was to simply shave the back rails on the receiver to match the undersized trunnion, and that was the quick fix...
The best fix is what you suggested, to get the correct rear assembly, but as noted, very expensive at this point, even if you could find it.