I know some of the GMC truck Hydra-Matic transmissions had external coolers (optional on smaller trucks, standard on larger trucks), but I've never heard of one being available on a car. On the trucks with coolers the connections for the fluid to enter and exit the transmission go through the side of the bottom pan and the pan is much deeper than the non-cooler pan. Your '55 Pontiac "slant pan" transmission would not lend itself to additional plumbing.
The torus in these transmissions do not generate as much heat as the torque converters in newer transmissions. I'd ask Jack Gifford his opinion of the need for a cooler. His GMC truck is equipped with the lightest (177C or 177CA, I can't remember which) HydraMatic being pushed by a MUCH more robust Pontiac V8 than yours. Jack is known for over-engineering almost everything.
https://www.facebook.com/groups/3864...4559186330594/ If he says put in the cooler, then I'd ask the transmission shop how they plan to attach the plumbing to the HydraMatic.