[Too Many Projects]

You previously said you had measured diagonally in different locations and distances and they were all "square", which would be a very good indicator that the frame doesn't have a diamond. This can be done as a rolling chassis even without it being on a perfectly flat surface.
Twist, is the only other concern and that would need to be done on a flat surface and measured from that surface to the reference marks in Tim's pic of the side view.

Knowing it measures square, I would feel fairly comfortable that the frame doesn't have a twist, or if it does, it's so slight, that it isn't going to make a difference.
If you want a little assurance, I would block and shim the frame under the front and rear corners of the horizontal rails until they are level, front to back and side to side. Then you can use a level on the front horns, rear boxed enclosure and a short torpedo level on the front crossmember to judge if they are twisted.

No frame is perfect, hence the need to shim the body to fit the frame on a Corvette.
The ONLY way to know 100%, is to take it to a reputable frame shop, with a printout of the specs, and have them check it. You could do this after it is assembled to roller status and it may easier to transport that way.