1969 Dart Pro touring daily driver **Crashed**

Nice progress.
You're also adding a lot of weight. Make sure you upgrade the Torsion bars to more manly sizes... ;)

As for the added crossbar, I'm not sure yet how you think it will help strengthening the frame rails if you keep it removable. Isnt' the crankpulley in about the same place as the bar?
I think a bit of creative work and an added homemade 'shell' over the existing lower radiator support would be more effective.

I would personally add a larger/thicker plate behind the support bars coming from the cowl.
It's only sheetmetal up there and won't do much in supporting the frame rails down below imo by itself.

As you can see on the white Duster Hotchkis project, they added a mounting plate with 'fingers' spreading out, making the area where the support bar are being welded, stronger.
Personally I would not use 'fingered' plates, but just a nice thick plate instead. I also would try to move the support bar mounting a bit more to the outside of the cowl box, where it will be more rigid near the corners.