I'm skeptical of anyone's "analysis" of the positions of political figures because the analyst's biases always color the analysis. That's obvious from both of the links you give: the "That's My Congress" link is obviously biased liberal just from their naming of the categories, and both of your links only analyze positions on a one-dimensional scale, which assumes that no other dimensions are relevant. That just increases the skepticism of someone (like me) who thinks both major parties in the US are seriously off the rails on some issues (for example, the war on drugs).

As for your list of "true" liberals, McGovern is the only one who strikes me as consistently furthering "liberal" positions (by which I mean "liberal" by the definition you appear to be using--see below). Mondale didn't strike me as consistently furthering much of anything. Johnson got us deep into Vietnam. FDR got us into World War II (by which I don't just mean he was President when it started, I mean he consistently took actions, often covert ones, that were calculated to get us into the war). Douglas voted with the majority in Korematsu v. United States (his positions did change over time, so, for example, he wrote the majority opinion in Griswold v. Connecticut; but that just points up the fact that he wasn't always consistent). Marshall did win Brown v. Board of Education, but while education in the US is more integrated today than it was then, I question whether it is any more equal, unless it's in the sense that it's more equally bad. (To be fair, Marshall was pretty consistently liberal in his opinions as a Supreme Court Justice.)

Of course, from your point of view, this just makes the case stronger that "true" liberals have very little power in the US. But from my point of view, it makes the case stronger that your definition of the word "liberal" is not a useful or historically accurate one.