Welding Tips and Tricks

I am rebuilding an older garden cart that was given to me many years ago by a close friend who recently passed. This cart has a unique tilting of the bed which makes it worthwhile to put the money and time in, besides I love to weld and rebuild things. Here is where I am having a problem. This is a support that will be welded to the new bottom I put in. The axle for the wheels rest on this. Using 1/8 mild steel. Top of support is 30 inch's long by 3 inch's wide. The legs I am trying to weld on to this piece are only 7/8 inch in height. I need to maintain a 90 Deg angle. So I talked the first leg on trying to use a welders magnet. The leg just isn't tall enough to grab the magnet so the leg moved inward on me when finished packing the 30 inch lenghth. Didn't put any tacks on the inside of the two pieces for fear that they would draw my angle in even more. Any advice would be much appreciated. Thanks, Mike.

Clamps, tacks, and tiny stitch welds would be my go to.

A light tack on the outside of the leg. Check for plumb. Correct with gentle taps from rubber mallet. Tack opposite side. Recheck. Hammer as required. Add tacks in other quarters. Correcting as you go. Once 4 tacked, you can weld it off. Checking as to half way around.

Metal moves. More heat, more movement. Thin metal moves like a sprayed roach. Welcome to welding.