Hellspawn, the OP just got a welder for Christmas and doesn't have, I assume, the experience and practice you do.
I think the advice rgvkid is giving is spot on; it was the same advice I received when I began welding and it was very helpful.
Is that a nail in your last picture? It looks like welding wire!
Measure the complete frame (two square sections) diagonally, if its unequal, measure each square section; hopefully only one and not both will be out of whack.
Great job so far!
Great work, Austin, looks very nice.
What about the roll bar behind the seat (page 33)? Looks as if the seat would interfere with it unless you slide it forward, but then you have less room.
The stop in the forward direction would be as far as the throttle and brake can be pressed. Rustydog is right, however, then you're relying on the hardware connected to the pedal to act as such.
I'm going to fabricate my own, it's "funner"!