Take a look at this page. It shows the location of the casting number on the transmission boss-
http://www.teufert.net/identify/identify.htm
If it does have the 9786135 casting number, that would seem to verify that you had a 4 bolt main, '67 428. Nice score.

Jim,
The casting number should be on the passenger's side/top where the tranny bolts to the block.
From the date code you have verified that it was cast Sept. of '66 and, if correct, that would definitely be a '67 model year engine.
The number below the YH won't tell you anything in that regard. It's the Engine Unit Number. Kind of an engine sequence number actually.
I THINK, at least for some years, because it looks like a "1", they didn't use an "I" and some they did...but I'm not sure.
I didn't get the pics?