^^^ Agreed. I would not personally be comfortable at all with a spare on top of a roof rack on top of a fixed high top on top of a 4x4 van, no matter what the top might be able to hold. Just too high of a COG for me.
Also, have you tried getting that on or off? Not gonna be fun.
If you want to know about what your top will hold climb up there and walk around. Most humans are anywhere from #150 to 250. See how it flexes, feels, sounds, etc. Preferably with a helper inside the van.