Setting up to weld square

[Red_boards]

I've read these threads and have a question about getting the vertical faces perpendicular to the base.
Will using magnetic braces be accurate enough (pic below) if the base is clamped to a flat base or is a firmer hold needed on the vertical faces to prevent distortion when the welds cool?

You can see the magnets in question (the BIG x-beams are underneath)


PS I've clipped the ends of the car in place. The "slots" cut in the end plates are diagonally opposite, rather than directly opposite - is this correct? I haven't seen any pictures where I can confirm for sure and the plans are silent on this (or my reading of the plans is!)

[Surfcnc]

Hi Red

Diagonally opposite.

To set the car straight Myself and AusMaddog and Skippy used threaded rod to set everything straight in the middle of the car.

Regards
Ross