Screwed in is interesting.... but it is not structurally as sound; one would expect the screws to work loose after a while with chassis twisting and bending. Not sure how Stitch weld; that will hold adequately. If you want a continuous weld, you need to do it in a few passes, with an inch every 3 inches and then go back and add another inch to each weld in the same succession. Trying continuous welds on sheet metal results in a lot of distortion in the parts. Access to the top of the trans IS good; maybe you could make up a smaller plate for that area.
