Sam,
I was there with the family at the grand opening, part of nice back-east vacation that year, but we had committed to coming so that we could see, and ride behind, PM 1225 as part of the festivities. Unfortunately as I recall very clearly and painfully the 1225 never made it there. The story was that it was determined at the last minute that it couldn't fit through the Detroit-Windsor tunnel on the way from Owosso to Scranton. I still have our 1225 excursion tickets (unused) from Steamtown as souvenirs.
Why do I mention this? I had spent some time looking at PM 1225's proposed route to the grand opening, trying to figure out how the 1225 crew might have gotten around the tunnel problem. Unfortunately I didn't do the same for MILW 261 -- primarily because it had been successful in getting there.
Sorry I can't be of more help, but here's a picture of 261 and the Big Boy, among other things, at the Steamtown site that day.
Mike