1. replace the blower motor. I recently saw a video fromthis guy:
and he replaced it with an American version from NAPA or such. Email him and find out. I wouldn't be surprised if the stock blower is whimpy. I did take mine out once to get water out and it looked strong when running. Who knows though. All a swap involves is widening the hole, bolt on the replacement and seal it well.
2. Also, forget that stupid plenum with all its leaks and inefficiencies. From the bulkhead hole, fabricate a smaller plenum leading to four tubes - two for the floor openings and two for the defrosters. You'll also have to fabricate flaps to control flow and reroute the cables but it should be worth it. This way you get all the flow to go through the tubes.
I would bet that if you did #2 you would not need a stronger blower motor.
3. You can replace the core with a more efficient aluminum one. This is most likely a custom job not done by you, hence $$$.