One detail that will really help the frame live is to NOT make the ends (front and rear edges) of the plate square and straight. Give them a full height curve and possibly at an angle as well. The frame wants to tear in a straight up and down direction, by making the edges of the plate cuvred or curved at an angle you don't put a stress riser in the direction that the frame wants to break.
Also, if you want/need 1/4" thickness you're better off laminating multiple plates (each of about the stock frame thickness) than going for it in one piece. Offset their lengths as you add each layer. So say the first layer is 14" long, then the next layer might be 12" long and so on. It's a LOT more work this way, but it results in a far stronger part.
Yup it's called fish scaling. I did that when I repaired some bad spots on my frame.
3/16" works fine too for plating the frame. That's what I used.
It's also far easier to use with a smaller 115 volt welder since you can get better penetration than using 1/4" but if you have a larger welder it's not really a issue.
