I am doing this right now on my Demon. I built my own boxes for long springs, bought Calverts spring perches(nice), and Afco sliders.
The factory spring mounts are not the most accurate layout and were designed for rubber bushings that could handle some misalignment. Do you want to use the factory mounts to locate your new stuff like the US Cartool stuff? If so that could be easier, just do yourself a favor and slip a rod through the spring holes from side to side to help line them up before you weld.
Save yourself some hassle,return the Affco and get the Caltrac sliders.They come with the right size bushings,,,,,,,,,,,,