The topic is interesting enough while just considering US built cars, but when you add in exported engines, it certainly adds more questions. As far as different suffix codes used for Canadian bound engines, that's perfectly normal to expect them, since the equipment itself was different. Perhaps at some point during the assembly process at Tonawanda, engines going to Canada were directed to a "side line" where they were dressed and racked differently.
One way to answer the question would be to look at the Oshawa plant AIM to see if there are assembly instructions for the bell housing and clutch cover. If those parts were already on the engines when they left Tonawanda, there would be no mention of them in the AIM.
Verne