The problem is when you start cutting the stock air box as much as needed for 20s you remove some of the inner circle that surrounds the blower cage. I,ve done them in steel and glass, never the same. the only why I know it works great is delete the stock system altogether and run a aftermarket kit like Classic Auto Air. Im actually fixing to do a fullsize C/K this way. You keep your stock condensor, and compressor out front, just adapt them to the evap case. Other cool advantage is a smooth firewall and a billet bulkhead that lines go thru.
Thats my .02