I have 89, so not sure how much of this applies. As you probably already know, there is a fixed amount air that enters the vehicle, based on speed the vehicle is traveling. You say that you're getting little to no air at the defrost vent and the floor. Both the defrost, and the floor vents are controlled by two separate climate control cables, so this is confusing. Maybe a silly question, but did you reconnect the climate control cables when you put the heater housing back in the van? Air towards the back of the van is fresh air (not heated), although cable number four should control this. (in an 89 at least) You can get at three of the four climate control  levers through the glove box area if you remove it. Even if the seals were bad, you would still get air flow,  it's just that when you close things off there would be some leaks. Sounds to me like the issue is cables. I can't imagine anything inside the heater box that would cause the restriction you're talking about. Closing the dash vent would help to distribute more air to the floor, but again, fixed amount of air, you only have control over where it's distributed, and that is controlled via the flaps within the heater housing, which in turn are controlled by the cables. Take a peek out through the glove box opening while you operate the climate control cables, and you should be able to see what's going on there.

Troy