U.S. Cartool Mini Tub kit..anyone tried it ?

I don't know about Darts, but I can tell you on Dusters and Barracudas that your tire will hit the springs before it comes anywhere near the wheel tubs. Do some measurements; you may not need to mini-tub to get the tire size that you desire. On my '71 Duster, I was able to run a 10.5" wide slick or a 305 50 15 street tire without having to tub, just by using the 3/4" spring relocation kit. The only welding needed for use of that kit is the welding of spring perches onto the axle housing.