even a carb for a 318 would be oversized for a 225 slant.
Many, many 225s have been easily made to run very well with 273-313-318 carburetors.
I understand the BBD on slants had a lower CFM rating than for a 318.
Yes, slightly, and some other differences—in 1977 to 1983, when the American auto industry wasn't interested in doing any better than a halfassed, barely-half-funded job of cleaning up exhaust emissions, so everything else went out the window. Performance, economy, driveability…pfft. The top number one priority was squeaking the new cars past their emissions type-approval tests so they'd be legal to put on the market, but they still resented being told they had to clean up the mess their cars were making of the air, so they rolled their eyes and pretended price and cost were the same thing. They could've bit the bullet and done a proper job of it; they sure as all hell had the engineering talent, but they didn't want to. And so there were umpteen different carburetor calibrations and giant amounts of money and customer goodwill pissed away on warranty work (and customers becoming former-customers) because the cars ran poorly.
Back to the case at hand: the car has the intake manifold and the Stromberg WW3 carburetor it has because the owner put them there (not a WWC3; that's the big 2bbl comparable to the big 1-1/2" BBD used on big V8s).
sometimes just like California vs US federal, where US federal was a year or more behind implementing certain requirements, Canada was often a year or 2 further behind in some of their requirements vs the the same model built for use in the US.
Canadian emissions standards stayed at the US 1974 levels until 1985, when they were made tighter, and then in 1988 they were fully aligned with the US standards. That's why many Canadian-spec cars could be bought new without catalytic converters through '84. Volvo would sell you a 240 with a single manual-choke carburetor through '84 (0 to 60 maybe sometime toward the end of next week), and GM allowed new-car buyers in Canada the option to say 'no thanks' to catalytic converters through 1979. It wasn't necessarily a good idea to do so; those cars were configured and strangulation-tuned like US '74 models, which were notorious for running like carp. Chrysler did a better job; they leveraged their Lean Burn technology to good effect, but then again, their catalytic converters were better than GM's, too. But none of this applies to the OP's '66.
they did put 2 bbl carbs on for some marine, industrial and export /6s could be had with a 2bbl carb before the "super six" came out on US cars in 77... I don't know what carb was used on those early non US version.
There were 2bbl packages for automotive and marine applications; industrial units running on gasoline got a 1bbl carburetor. The automotive package, released in 1967 in a bunch of markets not including the US or Canada, used a Carter BBD with an integral heat-tube choke, like
this one. The marine package, released a couple of years earlier, used a marinized but otherwise similar BBD.
...Once felt was oiled issue was resolved. Live & learn. Most carbs stumble on the off idle, hand off to an "accelerator pump" or pilot jet, that then hands off to Mains Jets/Needles/Metering rods, etc, once sufficient vacuum is obtained... to WOT. Just throwing it out there, Dont mind me.... 
