"Sounds about right except I don't understand "the center from the donor would be approximately 1" larger...." I had measured about 1/4" diameter difference."
Yeah, that would be my crappy memory, and not looking back at your post. Oh, well. Just looking for general ideas right now.
"the back spacing will need to be same as the 19.5" wheel used."
Yep,
Right again.

Good thing I sent a question, not an order. LOL
What's even worse is that when I said the backspace was about 9" I was thinking of the P30 wheel anyway. So I got it triple wrong.
Larry W.