I've never looked at the design, can't help there, but there may be something wrong with your setup, as I got plenty of defrost flow on my '73 P337. I live in a humid area very prone to condensation, so it HAD to work well for me.
Only problem I ever had was during maintenance, mechanics (embarrassingly including me) would disconnect the duct that bring air up to the windsheild on the right side, and forget to reconnect it, leaving the right side without any airflow at all.
Sorry I can't be more help.
Kevin
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