Ed-
I'm going to take a stab at this, but just remember--my clutch might be slipping a little because I'm trying to recall from a number of years ago, so take it with a grain of salt.
If I remember correctly, there used to be an old commercial FMVSS requirement of 250 CFM at the duct outlet; I believe this requirement was removed and the language was revised to allow anything that keeps the windows defrosted.
My guess would be if you used a similar size squirrel cage with the same horsepower motor & RPM, you'd be right in line with OEM defrost delivery performance. I know my defroster and vent ducts have very low delivery, which I'm attributing to a restriction in the heater core and A/C unit in the front box that you're getting ready to replace anyway.
Hope this helps and good luck with your project. I'll probably be doing the same thing in the next few months. . .
Art
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