I got to love the ingenuity of hot rodders!
I made some "gentle" curves similiar to yours in 1" sq. tubing and 1" channel some years back.. I made a simple 3 roll bender with 2 idler rollers very similiar to yours. I used a rim from a wheelbarrow for the drive roller in the center position. the rim had guides welded to it to center the tubing. By varying the depth of the drive roller in relation to the idlers I was able to achieve beautiful curves .
btw, I was building hand rails no hotrods,lol.