Snowracer, wouldn't welding all sides of your fish plate cause it to be in contraction "mode" after it cools and stress the welds? I'm no welder but I have heard about this before and it makes sense to me.
I also believe the reason for a "Z" cut is to keep all the vertical stress off of a vertical only weld, same goes for the fish plate to be welded on in a diamond shape instead of up and down.
P.S. your welds look awesome
I have a welding tips book and he does say that welding all sides is a no-no. Some people get away with it, some don't. The guy in the welding book says to always weld parallel with the frame. In other words, you can weld the two pieces of the frame together vertically, but on your plate only weld the top and the bottom of the plate, since the frame would rather expand and contract that way. Leave the vertical sides of the plate open.
One of the reasons they are starting to no allow frames to be welded is it weakens the frame, thus the reason you have to plate it. They stamp these frames out, and when they stamp them, it work hardens the metal and makes it strong, Even punching holes in the frame will make that area stronger. When you heat the frame during the welding process, it relaxes the metal and it's not as strong as it was.
