I had the same concerned when I swapped a 518 into my 87 Ram. It was replacing a 904 which, just like your car, had 5/16" lines and fittings into the cooler.

Raised the concern to my transmission guy. He couldn't advise me either way so he wanted to put a number on it.

Pulled the truck into his shop and disconnected my transmission lines. He then connected them up to his machine which measured the flow. Today I do not recall the number but he was more than satified with the Gallons/minute his machine was flowing through my cooler set up so he gave it his passing grade.

It was steel 5/16" lines into the factory radiator cooler. Rubber lines into a B&M stacked plate cooler. Back to steel lines, return to transmission.