The "Dash bar" as you so call it, isn't located inside the cabin. It's mounted against the firewall.The WHOLE floor has to be cut out anyways so Building "around a box" won't be necessary, and I'm short . Plus I have to replace a 1/3 of the firewall on the pass side.
I'm going to be building this off chassis, so I can be assured that this will be straight. But yes, I think just a simple box would be easier.
I'm going to be using a KA24DE so weight isn't really an issue.
I'm going off of 74_5.0L_Z design, somewhat. But looking at his pics some more. I think I h=need to add a little more tubing around the strut area.