A frame with heavy belting accross would conform to the shape of the tank. Many years ago, our boat dolly utilized surplus fire hose for the slings, were still in use when the cottage sold thirty five years later. Wasn't left out in the sun for extended time though.
Did you click on the part number for the 500 gal cradle and look at the blue print ??
Just remember, your tank when full of water will weigh over 3200 lbs, plus the weigh of the cradle.
No. Did not downloaded the PDF because i did not see the 400 gallon stand.
Looks like the only difference is length 84 vs 72.
I was thinking of building the frame out of 4 C channel, boxing the tank with the two cross pieces of channel holding the tank on the two flats it has on front and back.
With two more running a long the length of the bottom. And 12gauge sheet to do the seat.