When you spread a frame with a tool like that (or even a port-a-power) you are assuming that the frame spreads equally, it doesn't. The weakest side bends first. It's just my guess that on a Whizzer modified frame it will bend out first at the left stays where they have been Whizzer flattened, and the brake clip was welded on. Checking, and setting frame alignment is a good thing to do during the mockup step, "before it's painted".
With the frame placed in a sturdy vise like in your photo, just use a thin string around the frame head, and attach it to the flat sides of your fork ends. Measure from the string to the seat tube on both sides. That will tell you which side needs to be bent out. As you already know because of the metal spring back you need to "over bend it" and let it spring back. A six foot long 2 by 4 works perfect if you do not have the Park Alignment tools.
After you get the exact width, you desire and have confirmed it is in alignment "front to back", then you need to align the fork ends which will be out of alignment. The fork end alignment has to be checked and/or corrected as the last step.