I have Classic Auto Air in my Chevelle and Vintage Air in my Cutlass, planning on Classic Auto Air in the Impala cause I like the quality of parts and customer service much better.
Classic Auto Air has more efficient larger condenser and evaporator. Also their system has separate heat coil, like the factory-original A/C systems. Vintage Air combines the A/C and heat coils which is less effective.
I'm guessing the Magnum vintage air system is smaller due to these reasons. But there is no lack of space under the dash of my '67.