All GMT400's have a factory "cold air intake" where it does indeed come through the passenger fender, fed by a duct that is in the core support under the headlight.
On most, if not all, 1988-1993 models there is an extra piece of ductwork inside the fender that you can remove. Some feel it's a choke point as the opening of it is kind of small. GM quit using this part in 1994. Once you pull the resonator box and plastic adapter to the fender out of the way you will see the "lip" of a plastic piece -inside- the fender. There's a pull-pin style fastener inside of it, and there's one 7mm headed screw up towards the front near where the fender attaches to the core support. Pull the fastener and screw, then grab that plastic at its edge with vise grips and wrangle it out of the fender, then re-install the fender adapter and resonator box.
On the TBI trucks you can also eliminate the resonator by installing a full-length snorkel hose to the air cleaner. Some base model V6 trucks came with this part, and it was also sold aftermarket for a while (now discontinued.)
Finally you can eliminate the warm air flap inside the air cleaner housing, as long as you don't live somewhere that it gets really cold. A little bit of hackery to get it done and done right. Or, look for an air cleaner housing from a 2500/3500 model that doesn't have the flap at all. I scored one from a '95 C2500 5.7 a while back. Not sure what made that particular truck special where it didn't have the flap, suspect the less stringent emissions standards for the GVWR made it so.
Richard