How to Check Uni body for Straightness

I am working on repairing and stiffening my '69 Barracuda front frame rails and torsion bar cross-member. At one point in her life someone cut holes in the torsion bar cross member to install fender-well headers.

40796827851_fc94a7ddde_o.jpg20171223_140808 by shakedownscott, on Flickr

They also cut out this sheet metal piece below the firewall, that connects rockers to the frame rail.

26926809238_ddca7e9ce8_o.jpg20171223_140709 by shakedownscott, on Flickr

There is also some rust in a 10" section of the front frame rail right in front of the torsion bar cross-member.

26926809028_2e1792ecfe_o.jpg20180304_152249 by shakedownscott, on Flickr

Before stiffening these areas I want to make sure that everything is relatively straight. I have her on 4 jack stands two on the frame rails in front of the K frame. Then two in front of the rear spring mount on the rear frame rail.

I know the floor in my garage isn't totally flat or level. Laid a 4' level on the floor and there is a bit of a crown to the floor under the car. I have never driven the car so I don't know if it runs straight or has any real issues. I am a bit concerned about it potentially having some twist because it has a 440 in it combined with the holes cut for fender well headers.

Any suggestions on leveling her out and checking to she if shes square and straight?