Some additional information on fishplating the frame (a guy had a similar problem on a welding website) - not necessarily anything new on the first page, but if you select page 2, it does address recommended fish plate thickness, etc. Personally, in addition to the fishplating, I would have some holes drilled in the plate so as to have some holes to do some spot welds on also.
http://www.millerwelds.com/resources...sh-Plate-Welds
By the way - for any of you expert welders - since this is a repair of something scum inherited - i.e. there's no Z, do you recommend plating both sides - or maybe all 3 internal sides (include top/bottom insides??) Just a thought. The fact that it's not Z'ed would bug me til the cows came home...