I put 4 under my sons 67 Dart 270. Could NOT roll the front end at all. The rear was movable but not smoothly.

I took 5 of the $12 wood movers dollies, took 1 apart and put two more of the swivel wheels under 4 of them to make four each into 6 swivel wheel ones, and this works well.
The steel ones I use for my engine blocks. I just welded some steel strap on so the blocks sit square. They roll around real well.