two 18's in an ext cab truck.

Im thinking of upgrading from my current one 15 setup and was thinking about doing two 18's until I was told that two 18's would be too big for my extended cab Silverado 1500 truck. He said that the problem wouldn't be fitting two 18's in my truck but that the subs wouldn't have enough room to create sound waves and sound proper and it would be more of a one note hitter build. He said in my truck I would get more out of a smaller setup with multiple subs like 4 12's. Has anybody heard of this before? I was really set on two 18 inch hdc3's until I heard that so if anybody has some insight I would appreciate it.