DanielG, I think you have a valid point. I had a similar situation, at one point I started thinking about frame flex as I have an expensive aluminum radiator that was designed to be hard mounted to some welded on ears on the frame. I was concerned with eventually cracking the radiator so I did some testing. When I jacked up one front A arm the frame flexed more that .025 until the other front wheel came off the ground. My car has a K member and I found that the maufacturer just went from 4x3 to 4X2 steel for the K member. At the suggestion of an engineer friend that builds cars I decided to add an X brace to the K member. I made it out of 1 1/4" .120 wall DOM tubing and it took a couple of days.
After it was all in I did the same test again and the frame did not move at all. In fact when I jacked up the one side all the way the other side stayed on the ground and the whole side with rear end lifted off the ground.
I have seen a lot of frames that if I was building the car I would have X braced the frame between the seat area and X braced the K member.
This was how I did mine. One of the best decisions I made with building my car.
