Can i fit an oil cooler to a Chevy 350 engine?

Yes. There are aftermarket suppliers that offer oil coolers that use the oil pan's drain plug as an access point. INTO?

Many of the 350's have an external oil cooler. It was standard equipment on pickups with the towing package, and in fact it was a bit of a problem for a few years since the engine oil coolant line often leaked when everything else was still tight.