[JC455]

A.A.,
If it's anything like every other GM I've ever dealt with (and I'm sure it is), the cases will be completely different.
I tried to do this on a 1st and 2nd Gen F-bird- no go.
The A/C "suitcase" is very large- it's designed to house an evaporator, whereas the non A/C is just a duct to push air through. The non A/C cars gather their air from cowl vents and the air goes straight to the vents in the kick panels (I'm sure someone will go into the finer points of A/C air control if you need help, so I'll just stay with this for right now).
Now, your problems happen because:
1) The A/C system has a duct that diverts
hot/cold air into the plenum for distribution, but the non air cars only run air to the heater core.
2) The large hole for the A/C diverter flap will not be covered by the small non air cover
3) The holes for the non air box will not line up with the "suitcase" mounting holes.

What I did was cut (well, actually someone broke it) the suitcase down to a level I felt was adequate enough to pass a large volume of air through. Then I used a piece of cardboard to fill the gap across the front and fiberglassed the cardboard to the suitcase. After some sanding and painting you couldn't tell it wasn't a factory item.
By doing it this way, I still had a heater and a fresh air vent. Of course I removed all the a/c parts I didn't need.
Now granted, this is not the best method to convert to non air, I just didn't feel like doing a factory-non air-resto job.
I hope this helps you, if you have any questions, please post them. I'll try to clear any confusion I may have created.


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John