Moved to "Hudson" forum category.

The "boxy" looking tank is correct for '51.

As Park said, the "boxy" tank is correct but if you are seeking a replacement, my opinion is that it's advisable to get the later, rounded tank style. They are interchangeable. The early flat sided ones with stiffening indentations do a lot of growing and shrinking with each heat/cool cycle and eventually crack at those ribs. You'll hear a few "boink!" sounds during warm up and more "boinks!" during cool down. It can only take so much of that repeated flexing. Sure, you can solder 'em up, but it will soon repeat the problem.
Frank

Agreed, Frank. If original appearance isn't an issue, go with the "rounded" tank.

Not to beat this subject to death, but.......
I wanted to replace my '51 radiator due to the aforementioned issues (it showed signs of several previous repairs at the subject areas) so I sought an Aluminum unit but couldn't find anything less than a custom built, very expensive one. I looked at many but decided on one spec'd for '55-'59 Chev pick-up with some mods to make it fit the Hudson. I specified that the 1-1/2 inlet be placed at 6 inches left of center (toward driver's side) and that the 1-3/4 outlet be modified with a 80 degree, upward angled fitting.
I chose this particular mfgr. because they form the top tank with ribs that mimic the original tank's appearance, however, they are cosmetic and not necessary for stiffness. It came with a 1.1 bar cap which I replaced with a 7 lb. cap. (~.5 bar).
Although not a direct fit, it was a fairly simple job to adapt it to the Hudson radiator support. Cost was ~$500 total.
F

The round top radiators are '52 and later, but the old timers and race car drivers preferred the flat top radiators and would have them put in. I prefer the flat top radiators. I'd say, go with the flat top one, it's correct for your car and it's what all the cool kids have. I've only heard they are better and I have them in my '52s. If you don't, then more for me!