Definitely use a spot weld cutter. Using a chisel will distort the living heck out of the metal under the channel. I use them frequently and have started to use a Blair cutter. They are very high quality and come in a small blow molded case with three cutters, a shank, and an extra center pin. Sharpen your punch to needle sharp when center punching. Removing the paint will really reveal the spot welds as will a light shown from the side.

If you can please show lots of pictures. I have considered making a new metal top. It would be cool to see what you have.