I wouldn't have a tank that big not be firmly planted on something.
I've been getting away with having a 10 gallon on something that is not quiet as long as a 10 gallon, but I wouldn't have a hang over with a 75. I feel like its asking for the tank to bust open or warp over time.
Besides, would be a pita to get a proper stand after setting up the tank, so I'd just get a stand now.
If all 4 corners are solidly and evenly supported, it will work fine. Because the 75 is deeper than the 55, I doubt this to be the case. If any one corner is not fully supported, the tank will eventually fail.
I won't put it on the stand if it isn't fully supported, I was curious as to if it will fit. I found a good deal on a 75 and would like to replace my 55 but right now I can't get to my 55stand to measure it. Sorry I didn't put all the info in the op