Maybe replacing those southerner US sounding actors with some from the British pack(s), and add a couple with French accents?
Canadian accents are quite different from British accents - except for immigrants. French - canadian accents are also different from the continent. English canadian accents are closer to generic American actors or newscasters. As a matter of fact a lot of them are canadian.
I am aware of the Canadian accents, living in Toronto for the past 15 years or so, thankyouverymuch
and yes today it is much more alike to American accents (with regional oddities in Maritimes and especially Newfoundland - which, back during WW2, wasn't really Canada but a separate nation). I was counting on the fact that back then there was a greater percentage of immigrants from Britain in Canada compared to today, and also if you listen to the recordings of radio broadcasts you can hear that CBC was really pushing the "Queen's English" accent during this period.
From what i see, having something a mix of something like 60% of US voices, 20% of British and 20% of English with French accent (which would then be the only ones that would need to be created from scratch) could do the work.
We could also go through the trouble of making a full voicepack with current Canadian accents, but i'm not sure it would be worth the effort - and it wouldn't be necessarily any more historically correct than using a compilation of existing voices + a few new ones as i suggested earlier.