To answer your question about converting to a coil spring, it's rare to see it done. you odn't ave much room to work with and no one makes a kit for it that I know of.

as far as the SAW torsion bars go, Sway a way makes a great and product and I highly recommend them.

They are also available here:
https://ruggedrocksoffroad.com/nissan-xterra-wd22-n50-torsion-bars-c-42_1181.html

Post up if you have any further questions.