It's simple to do and doesn't require good welding skills or additional tubing. Cut the two cantilever tubes away from the down tube and away from the seat tube. Then using one of those ratcheting tie-down straps, wrap it around the seat tube and the rear dropouts and tightened it up until the seat tube is pulled back and resting on the rear fender/brake brace that's between the rear stays. Where all the tubing is now making contact weld it back together on the underside of the bike and then cut off the extra cantilever tubes that are now extending past the down tube. Leave the bearing cups and crank in place when manipulating the bike. The bottom bracket is the weak spot and will collapse instead of the rest of the bike bending. It makes a subtle change in the look of the bike and gives you the option of a low-rider because it actually lowers the bike or add chopper forks long enough to get the bottom bracket back up to normal height.