heater box modification info wanted

[foxxinabox at wideopenwest.com (Foxx (in a box))]

   i'm going to replace the heater core in an extra box that i have 
laying around. i know the evaporator is good so i'm just going to 
replace the whole unit into in my '86. i thought i saw somewhere that a 
modification could be done to get warm air to blow through the side 
vents. does anyone have that information? does effect the a/c blowing 
through? thanx.

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