You can see above how the penetration has come all the way through the join without being excessive either. Too much just adds to un-necessary distortion without strength gain. I even ground and sanded the welds while still attached to the heavy wall tubing to keep it straight. In the end the straight rear 2/3rds of the frame only ended up with a 1/2" dip in it. I gently pressed that out until straight again.
The rails were completely sanded with a 7" disc sander to get it as flat as possible. Started at 16 grit and worked my way down to 80 grit. Any pitting from old rust was then spot blasted with garnet to remove the last signs of rust.
Making the rear cross member. The stock one was 52mm-2" wide x 3mm-1/8" wall. As I wanted to use it as a towbar as well, I increased the width to 76mm-3" by using two cross members both trimmed evenly to make that width. The edges were all bevelled and a gap left to get full penetration. Can see also that it was set slightly up, so when the weld shrank, it would pull it down flat and square.
The receiver was fully welded in as well. The part protruding will be the back of it with the front face left flush for licence plate mounting. The receiver tube was flush with the inside of the cross member so a fillet weld could be used to give the greatest strength.
As you can see the weld pulled down the join so it was nice and flat. It was welded in sections to balance the heat between the top and the bottom seam to prevent any bending. Tabs for the safety chains were also added.