Welcome to the forum.....and you have an ugly situation. If you cannot absolutely get to the bolt heads above then there are not a lot of options unless someone else can suggest something clever.
So I assume you have no space between the head of the bolt and the nut you are trying to remove and the nuts are frozen onto the bolts (probably from road grime exposure). Start with some PB Blaster or other rust release spray on the nuts. If there was some space above the nut you could try prying down on the nut with a flat screw driver or other flat tool to put the bolt head in tension and try to break the nut loose with a wrench while prying down on it. This is subject to slippage and not a great option.
If there is enough of the bolt threads exposed below the frozen nut, you might try another method below (you can’t grab the bolt with vice grips or you will damage the threads and sounds like you won’t be able to replace the bolts from above). The other approach if there is enough bolt below is to use two appropriate nuts and thread them on below and jam them together (tighten them onto themselves) but not against the frozen nut. Try and break the frozen nut loose with a wrench while holding the jam nuts with a wrench or vice grips. That is a PIA situation but under the steps is a double PIA. My sympathies.
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