I have lines going to radiator(radiator was from 318 A/C car, stout radiator) Why would I want an aftermarket cooler ? I do have a shift improver kit and 300 stall convertor and may "dog it" on occasion - will aftermarket be more efficient ?

Its generally best to run the oil through the radiator cooler first and add an additional cooler in series after the radiator cooler if needed. If you're running a 318 with slightly over stock stall (does "300 stall" mean 300 over stock?) I wouldn't think you would need an auxiliary cooler.

A little more information about your combo would help (engine, trans, convertor, rear gear, how you'll use it).

Thanks - rebuilt "warmed over" 340, 3.91 rear, to the track 3-4 times a year - just don't want issues in traffic ....

factory coolers in bottom of rad do not have enough area to do much. Some add one in series,I prefer to keep coolant & ATF seperated & just use a large aftermarket one by itself.

I would think in your application the stock style would be fine. Easy to add one later if you have high temps... browinish burnt smelling ATF. But I'd think you'll be OK.

I recommend an after market cooler + the radiator cooler. Even OEM tow packages or extra cooling packages came with the external cooler installed as the self contained cooler was not deemed efficient enough. Once you have discolored smelly fluid it is too late.