i have a 1957 Chevy Two Ten similar to the Bel Air, which has a 350 engine and a 350  transmission and  stock radiator, which the oil from the transmission goes into.    Would it be better to use a separate oil cooler for transmission oil than using the stock radiator to cool the transmission oil?  

You realize your tranny fluid is not just loose in the antifreeze right?  It is a sealed system that routes the trans fluid through the radiator for heat transfer rather than just using air-cooling like an external cooler would offer.  The radiator method is your best bet unless you go to a higher stall converter, in which it would be recommended to supplement your radiator set-up with an extra trans cooler, but not exclusively.  

yeah, i know that it is separate, just not about the higher stall converter, and thank you for your help, and quick response i think I'll just keep the radiator  

If you have more than 370H.P. then I would add a trans. oil cooler but run it were both oil coolers work together.  

Any time you can add to fluid capacity or cooling of the fluid it will aid in the longevity of any transmission.