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I've been thinking that there shouldn't be any problem with lining an Ultramatic Transmission Oil Cooler with a sealer such as POR 15 as long as the main body is cleaned thoroughly and liner isn't allowed to build up too thickly on the internal fluid lines. Heck, after 60 years or so, those lines have an accumulation of calcium and rust build-up on them anyway and they're still dispersing heat. Anyone see a problem with it?

Yes. any impairment to heat transfer, in my opinion, is not good.

Shouldn't be any rust on the heat exchanger itself, it's stainless steel. Could be some scale I suppose from neglectful prior owners but a simple visual inspection should reveal that. Trying to save the mild steel outside casing by coating it is a good thought but since you won't be able to keep the coating off the heat exchanger itself, I think that in the final analysis it's probably not a good idea.

There is I believe a reason why we so often find the necks badly corroded and sometimes perforated, other than that Packard didn't consider their need to last a half-century. I think it is exacerbated by the location of the hose clamps. If the clamp is well down on the hose and neck towards the cooler body, the hose will bellow a bit from the clamp tension and system pressurization and there remains a static cavity for coolant to lay dormant and once the corrosion inhibitor is exhausted (2 years?) you can get attack of the neck from the outside above the clamp location (which is the common observation). At least that's my theory. I think the clamp should be as near the end of the neck as practical. And since this area is the most vulnerable, I think your idea of using POR is a good one, but I'd limit it to the inside and outside of the two water necks.

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I agree on all counts Owen. I just had new ends welded on to my existing cooler made out of mild stainless exhaust tubing. I suspected that there were most likely voids in the butt welds where they were joined so I wiped the welds and the remaining tubing on the inside with a coating of epoxy. I also used epoxy to fill a couple of pin holes in the body so it should be good for a few years yet. Meanwhile, I've located a good spare cooler that will allow me to keep the old gal going long enough to outlast me.