97 Taurus Rear Spring upgrade for heavier load capacity?

I am planning on replacing the rear struts and springs on my 97 Taurus GL 3.0L soon. I haul a lot of tools and even a trailer for motorcycles on the occasion, so I'd like to put heavier springs in the back. The standard springs for the sedan, which is what I have now (although worn) will not hold up well to the loads I need to carry. The Taurus wagon springs are variable rate and made to handle more load to the best of my knowledge. The Ford dealer told me they would fit on the sedan strut, but they cannot verify if it will modify the ride height. If the wagon springs raised the rear slightly, it wouldn't be an issue. However, if the ride height was raised significantly, that would not be an acceptable option. I found custom springs for the sedan that are heavier than OEM, but they are very very expensive and I'd rather not go that route.

I am running stock springs in the front with new KYB-GR2 struts and plan on replacing the rears with the KYB as well.

Any suggestions?

Thank you.