Hey Sam,
Those welds look OK. For your own satisfaction spend a bit of time welding similar things that you are using on the car and then giving them hell with a BFH. You should be able to completely mangle a piece of 1&3/4 tube which you have welded to a rail. If you crack the weld or break it out along the bead line then it is of less than ideal strength. First you need to satisfy yourself that you are getting full penetration, then you need to work on tidier beads, tidier beads have less potential for crack propagation. Many people will say " the neater the bead the better the weld", if the penetration is good" that's pretty close to right....
You're right about "putting it out there" too. I'm one for putting all of your exploits in the public view, if there's something wrong you'll know about it way sooner than Secret Squirrel will with his well hidden poorly conceived ideas, you'll learn way faster and get stuff done quicker.
Is that gasless you're using?.....it looks very spattery, are you keeping your torch clean? Consider some inox spray to save it sticking everywhere.
My only suggestion is to get in a bit of practice on mismatched thicknesses. It's much easier to control heat and penetration of two identical pieces, that changes when you are welding light to heavy and the risk of poor penetration on the heavier guage occurs, the piece of tube onto the rail above could maybe do with a little more wire speed , less "cross-stitching" and more focus on the rail than directly at the root of the joint. I say that because the weld has undercut the tube slightly, which contrasts with the edge of the bead on the rail, "cross-stitching" will allow more fill and dissipate heat but has left a ragged finish to the bead, by allowing the bead to flow out a little more it will run a little hotter, focusing that heat more at the rail than the root of the joint will increase the penetration on the rail, lessen the burn on the tube and give a better finish to the bead.