The front section of the longeron is twisted and comes down at an angle, thus if those things were welded out to the end, they would not fit the inside of the longeron. That is why they are not weled out to the end.
Contratulations, Phil, you have discovered the "Oh, No, the firewall weldements look like they are cracked" easter egg. Proceed to next step, and keep your eyes open for the next one. Van has planted quite a few of these for your enjoyment.