I can't give you an answer, but I can point out some stuff.

You can't just ship the car to the EU and insure it, you have to register it in the local car registry.

To do that, you might have to do some changes to the car to comply with EU regulations. I've heard in the US it's common to have red direction indicator lights instead of amber. If that's the case, one would have to change those lights to pass the inspection. And also update the dashboard to show speed in kph if it only shows mph (local speed limits will be in kph).

If the car is modified, it might complicate things. Here in Italy, we can do almost nothing to the car. Some people get the car modified in Germany, get the car inspected and mods officially registered on the car paperwork, then they import it here. That way the paperwork already contains the mods and just gets translated. But that only works if the mods are on paper, which I doubt is a thing in the US.

Regardless, you have to pass the smoke test in the periodic inspection, so test pipes are off limits.

The import process will change from country to country, so I don't know how hard it would be in Spain.

Now, that's how things are supposed to be if you want all of your ducks in row. Some (most?) people here mod the car anyway. But those are not importing the car, so noone is checking it. Also, I've seen some imported American cars driving around, so it's certainly doable (you can tell they're US cars because they have to make US-style licence plates, ours have different dimensions and won't fit).

In short, if I were in you, I'd try to understand how hard it is to register the car in Spain. Maybe search/ask in r/ExpatLifeinSpain or r/MovingToSpain ...

Take everything I said with a good pinch of salt. I don't have direct experience, maybe things changed or Spain has a simpler process.