Unfortunately, because the cowl vent holes are pretty small, you don't get a lot of air against the windshield. I did a bit of experimenting and didn't notice much difference in how much air came through, even with different heater-case door sealings. It's just part of the "culture shock" that we live through after experiencing the blast furnace that some contemporary cars throw...
I remember a magazine description of an MGB heater from back in the '60s:
"It sounds like a blast furnace but feels like a little old lady breathing on your leg."
