Got some upgraded pix from the usual doodles: MSPaint!
How I plan to notch the frame:
How I plan to fish-plate it after welding the seam on both sides:
A diagonal brace I plan on using (like the one I did in the rear) to tie the angle together:
Super crappy attempt at a 3D drawing showing an x-brace welded to the diagonal brace shown above:
If all that looks acceptably strong, the next question I have is how much to notch? I would love to make a nice straight cut, lower the frame down until the seam is touching, then weld it up. This is where I need a math whiz lol.