I haven’t seen your wife’s more generous rear so I have no idea if it will fit a blackline.
In case you were asking if the seat will fit your bike the answer to that is drag out that Harley catalog and look a the slim sundowner and see if your blackline is listed as well. I think you will find the seat will fit all Softails of the same frame vintage.
China be right, Ferrett if you ever want to offload the original blackline seat let me know.
I already know her rear doesn't fit (pity it was a thing of beauty in her younger days) what is the weight so I can get an idea of post, money is tight but if it is still here I will find some.