Oldsmobile Omega A/C retrofit with evap before blower motor

[V8Power]

I'm looking to retrofit my 1980 Oldsmobile Omega with A/C. I bought a complete system from an 80's square body. The compressor, condenser and hoses will fit fine. The big problem is the evap core. The only way I can integrate it into the original system is by putting it in front of the intake to the blower motor. The heater box doesn't have provisions for an evap core.
Is the area of about 4x4 inches enough to cool the air somewhat or do I need more surface area?