The tanks are the same - until you've used it!
The only thing that might preclude you from putting it in the floor is if your toilet is in a "corner". At least with our trailer, the belly pan/banana wrap rolls up along the perimeter so the depth isn't there for the tank within the frame there. The tank would have to be way too shallow or wedge-shaped. Depending on which tank you are looking at, I think they are going to be deeper than your 1958
frame so they will need to either stick up or drop down anyways unless you have them custom made (we did because they weren't available at VTS at the time).
We opted for an above floor tank and a low-boy toilet. The toilet really should sit directly above the tank - while it doesn't have to be centered, you really don't want any horizontal travel which could cause blockage in the "poop-chute".